Gui: Provide ViewProviderFeaturePython selection callbacks

Adds support for Python-based selection callbacks to
`ViewProviderFeaturePython` objects.

It follows the same conventions as `SelectionObserverPython`, follows an
example:

```python
    def setPreselection(self, document, object, element):
        print("setPreselection: %s.%s.%s"%(document, object, element))

    def removePreselection(self, document, object, element):
print("removePreselection: %s.%s.%s"%(document, object,
element))

    def addSelection(self, document, object, element, position):
print("addSelection: %s.%s.%s at %s"%(document, object, element,
str(position)))

    def removeSelection(self,document, object, element):
        print("removeSelection: %s.%s.%s"%(document, object, element))

    def setSelection(self,doc):
        sel = FreeCADGui.Selection.getSelection(doc)
        print("setSelection: %s"%sel)

    def clearSelection(self,doc):
        print("clearSelection\n")
```

src/Gui/ViewProviderFeaturePython.h

@@ -26,17 +26,19 @@
 #include <App/AutoTransaction.h>
 #include <App/PropertyPythonObject.h>
 #include <App/FeaturePython.h>
+#include <Gui/Selection/SelectionObserverPython.h>
 
 #include "ViewProviderGeometryObject.h"
 #include "Document.h"
 
-
 class SoSensor;
 class SoDragger;
 class SoNode;
 
 namespace Gui {
 
+class SelectionChanges;
+
 class GuiExport ViewProviderFeaturePythonImp
 {
 public:
@@ -55,6 +57,7 @@ public:
     QIcon getIcon() const;
     bool claimChildren(std::vector<App::DocumentObject*>&) const;
     ValueT useNewSelectionModel() const;
+    void onSelectionChanged(const SelectionChanges&);
     ValueT getElementPicked(const SoPickedPoint *pp, std::string &subname) const;
     bool getElement(const SoDetail *det, std::string &) const;
     bool getDetail(const char*, SoDetail *&det) const;
@@ -130,6 +133,7 @@ public:
 private:
     ViewProviderDocumentObject* object;
     App::PropertyPythonObject &Proxy;
+    SelectionObserverPythonHandler selectionObserver;
     bool has__object__{false};
 
 #define FC_PY_VIEW_OBJECT \
@@ -253,6 +257,10 @@ public:
             return ViewProviderT::useNewSelectionModel();
         }
     }
+    /// called when the selection changes for the view provider
+    void onSelectionChanged(const SelectionChanges& changes) override {
+        return imp->onSelectionChanged(changes);
+    }
     bool getElementPicked(const SoPickedPoint *pp, std::string &subname) const override {
         auto ret = imp->getElementPicked(pp,subname);
         if(ret == ViewProviderFeaturePythonImp::NotImplemented)