Restructure Canvas/PropertyBasedElement

Restructure Canvas/PropertyBasedElement

Nodes inside the osg scenegraph now hold a strong reference to
the according canvas element. Canvas elements in turn now only
hold a weak reference to the according node. This simplifies
for example the canvas::Group as it does not need to keep a
list of children on its own anymore. This is now stored inside
the scenegraph (as it was already before).

The restructuring will also allow to use a canvas::Group for
the canvas gui inside FlightGear and share for example the
handling of stacking based on z-index.