Some of these new APIs are not strictly needed for Link to work, but
good to have.
Summary of the API changes:
* getModeSwitch/getTransformNode() exposes view provider mode switch
and transformation coin3D node, required by ViewProviderLink to
override placement and visibility.
* canAddToSceneGraph() inform 3D viewer whether to add the root node to
scenegraph. Some object only exists as a child of some coordinate
system. Not adding them can simplify scenegraph and visibility
management.
* showInTree() inform tree view whether to show the corresponding tree
item.
* getDefaultMode() to expose the current active mode regardless of the
view provider's visibility
* (can)DropObjectEx() superseds (can)DropObject() with additional
support of subname reference, which makes it easy to support linking
to sub-objects. One of the use case is cross coordinate system
linking.
* getDropPrefix() is used to tell tree view where the object is dropped
into. A non empty return means the object is actually dropped into a
sub-object. For example, Assembly3 container puts all dropped object
into its child container PartGroup.
* canDragAndDropObject() is used to inform tree view whether the object
dropped need to be dragged out of its original parent, which usually
does not make sure for Link type object.
* beforeDelete() will be called by Gui::Document when either the
object itself is being deleted or when the document is being
destoried.
* is/setLinkVisibility() allows to show/hide a link to this object.
This may be used during editing, to prevent showing editing geometry
in multiple places.
* update() is made a virtual function, it will be overridden by
ViewProviderDocumentObject in future patch.
* startEditing() now becomes virtual, and return a ViewProvider that
actually handles the editing. This is for future Link type object to
forward editing request
* covert(), convenience function to convert between Coin3D and FC matrix
ViewProviderExtension also gains many relavant extension point of the
new API. There is also the new extensionModeSwitchChange() for notifying
mode switch changes
ViewProviderPy exposes several method/attribute for the new API as well.
FreeCADs property system utilises some pointer math to calculate the offset between
property and base class. Due to virtual inheritance of th ePropertyContainer the memory
layout has been changed to rather random, which has lead to crashes dependend on the
order of object initialisation.
The solution is to not make PropertyContaner virtual but a class below, Base::Persitance.
Then the memory layout is random for Persistance, but it is perfectly aligned for the
base class chains from PropertyContainer onwards as well as from Extension onwards.
Hence the proeprty system was changed to take the offset always from those two.
