ViewProvider(DocumentObject): new APIs for context aware selection

Context aware selection makes it possible to select the same Coin3D node
in different hierarchies (i.e. context) without ambiguity.

New/modified APIs in ViewProvider (the first two are the most crucial
APIs for context aware selection to work):

* getElementPicked(), supercedes getElement(). Given a Coin3D pick
  point, this function returns a dot separated subname reference as a
  path leads to the selected object.

* getDetailPath(), supercedes getDetail(). Given a subname reference,
  this function returns an SoFullPath leads to the Coin3D node of the
  selected object or sub-element (with SoDetail).

* (has)HiddenMarker(), check/return a special text marker for context
  aware override of object visibility.

* partialRender(), render only part of the object based on given
  subname references. It can render, e.g. some faces of a solid, or
  some children of a container. It uses the 'secondary' extension of
  SoSelectionElementAction to select which elements to render or hide.
  The actually rendering functionality will be added in the following
  patch.

* update()/onChanged(), modified to sync Visibility property from
  DocumentObject.

* useNewSelectionModel(), modified to return the default true view
  parameter. This is for test in the early stage of Link development,
  probably not needed anymore.

* getBoundingBox(), a convenience function to obtain the bounding box
  of a sub-object/element regardless of object's visibility. It uses
  getDetailPath() and SoGetBoundingBoxAction to obtain bounding box
  through Coin3D. It will be used in later sub-element box selection
  functionality.

New/modified APIs in ViewProviderDocumentObject:

* getElementPicked()/getDetailPath() provides actual implementation to
  support container like (sub)object selection without ambiguity. It
  relies on DocumentObject::getSubObject() to walk the path.

* reattach(), called when undo deleteion

* forceUpdate()/isUpdateForced(), force update even if object is
  invisible. These are used by Link to force update the visual of a
  linked object regardless of its visibility.

* getLinkedViewProvider(), return the linked view provider with
  hierarchy.

ViewProviderDocumentObjectPy:

* Object attribute is made writtable. Assigning it is equaivalant of
  calling ViewProviderDocumentObject::attach() in Python.

src/Gui/ViewProviderDocumentObject.h

@@ -24,9 +24,10 @@
 #ifndef GUI_VIEWPROVIDER_DOCUMENTOBJECT_H
 #define GUI_VIEWPROVIDER_DOCUMENTOBJECT_H
 
+#include <Inventor/SoType.h>
+
 #include "ViewProvider.h"
 #include <App/DocumentObject.h>
-#include <Inventor/SoType.h>
 
 class SoMaterial;
 class SoDrawStyle;
@@ -59,9 +60,12 @@ public:
     // Display properties
     App::PropertyEnumeration DisplayMode;
     App::PropertyBool Visibility;
+    App::PropertyBool ShowInTree;
+    App::PropertyEnumeration OnTopWhenSelected;
 
     virtual void attach(App::DocumentObject *pcObject);
-    virtual void updateData(const App::Property*);
+    virtual void reattach(App::DocumentObject *);
+    virtual void update(const App::Property*) override;
     /// Set the active mode, i.e. the first item of the 'Display' property.
     void setActiveMode();
     /// Hide the object in the view
@@ -69,6 +73,13 @@ public:
     /// Show the object in the view
     virtual void show(void);
 
+    virtual bool canDropObjectEx(App::DocumentObject *, App::DocumentObject *, 
+            const char *, const std::vector<std::string> &) const override;
+
+    virtual int replaceObject(App::DocumentObject*, App::DocumentObject*) override;
+
+    virtual bool showInTree() const;
+
     /// Get a list of TaskBoxes associated with this object
     virtual void getTaskViewContent(std::vector<Gui::TaskView::TaskContent*>&) const;
 
@@ -83,6 +94,22 @@ public:
     /// Get the python wrapper for that ViewProvider
     PyObject* getPyObject();
 
+    /// return a hit element given the picked point which contains the full node path
+    virtual bool getElementPicked(const SoPickedPoint *, std::string &subname) const override;
+    /// return the coin node detail and path to the node of the subname
+    virtual bool getDetailPath(const char *subname, SoFullPath *pPath, bool append, SoDetail *&det) const override;
+
+    /* Force update visual
+     *
+     * These method exists because some view provider skips visual update when
+     * hidden (e.g. PartGui::ViewProviderPartExt). Call this function to force
+     * visual update.
+     */
+    //@{
+    virtual void forceUpdate(bool enable = true) {(void)enable;}
+    virtual bool isUpdateForced() const {return false;}
+    //@}
+
     /** @name Restoring view provider from document load */
     //@{
     virtual void startRestoring();
@@ -96,6 +123,20 @@ public:
             const char* group=0, const char* doc=0,
             short attr=0, bool ro=false, bool hidden=false) override;
 
+    /** Return the linked view object
+     *
+     * This function is mainly used for GUI navigation (e.g.
+     * StdCmdLinkSelectLinked). 
+     *
+     * @param subname: output as the subname referencing the linked object
+     * @param recursive: whether to follow the link recursively
+     *
+     * @return Returns the linked view provider. If none, it shall return
+     * itself.
+     */
+    virtual ViewProviderDocumentObject *getLinkedViewProvider(
+            std::string *subname=0, bool recursive=false) const;
+
     virtual std::string getFullName() const override;
 
 protected: