Sketcher: Show attachment overlay icon information

==================================================

ViewProviderSketch now derives from the ViewProviderAttachExtension and gets the overlay icon when not attached.

ViewProvider2DObject does not implement a ViewProviderAttachExtension although Part2DObject (on which SketchObject derives)
does derive from AttachExtension. It is understood that this is because this functionality is unwanted for other
ViewProviders.

src/Mod/Sketcher/Gui/ViewProviderSketch.cpp

@@ -301,6 +301,8 @@ ViewProviderSketch::ViewProviderSketch()
     isShownVirtualSpace(false),
     listener(0)
 {
+    PartGui::ViewProviderAttachExtension::initExtension(this);
+
     ADD_PROPERTY_TYPE(Autoconstraints,(true),"Auto Constraints",(App::PropertyType)(App::Prop_None),"Create auto constraints");
     ADD_PROPERTY_TYPE(AvoidRedundant,(true),"Auto Constraints",(App::PropertyType)(App::Prop_None),"Avoid redundant autoconstraint");
     ADD_PROPERTY_TYPE(TempoVis,(Py::None()),"Visibility automation",(App::PropertyType)(App::Prop_None),"Object that handles hiding and showing other objects when entering/leaving sketch.");

src/Mod/Sketcher/Gui/ViewProviderSketch.h

@@ -25,6 +25,7 @@
 #define SKETCHERGUI_VIEWPROVIDERSKETCH_H
 
 #include <Mod/Part/Gui/ViewProvider2DObject.h>
+#include <Mod/Part/Gui/ViewProviderAttachExtension.h>
 #include <Mod/Part/App/BodyBase.h>
 #include <Inventor/SbImage.h>
 #include <Inventor/SbColor.h>
@@ -84,7 +85,8 @@ class DrawSketchHandler;
   * of new geometry while editing.
   */
 class SketcherGuiExport ViewProviderSketch : public PartGui::ViewProvider2DObjectGrid
-                                           , public Gui::SelectionObserver
+                                            , public PartGui::ViewProviderAttachExtension
+                                            , public Gui::SelectionObserver
 {
     Q_DECLARE_TR_FUNCTIONS(SketcherGui::ViewProviderSketch)
     /// generates a warning message about constraint conflicts and appends it to the given message