Port Attacher codde to the extension framework

AttachableObjects are desired in multiple occasions, and the current AttachableObject is not flexible enough to handle all cases. Hence the code is portet to an extension, which gives the needed flexibility.

src/Mod/Part/App/DatumFeature.cpp

@@ -32,10 +32,11 @@
 using namespace Part;
 using namespace Attacher;
 
-PROPERTY_SOURCE_ABSTRACT(Part::Datum, Part::AttachableObject)
+PROPERTY_SOURCE_ABSTRACT_WITH_EXTENSIONS(Part::Datum, Part::Feature)
 
 Datum::Datum(void)
 {
+    AttachExtension::initExtension(this);
     touch();
 }
 
@@ -47,7 +48,7 @@ void Datum::onDocumentRestored()
 {
     // This seems to be the only way to make the ViewProvider display the datum feature
     Support.touch();
-    AttachableObject::onDocumentRestored();
+    Part::Feature::onDocumentRestored();
 }
 
 TopoDS_Shape Datum::getShape() const