STYLE: Use properties in CreateExtensionDialog

Change CreateExtensionDialog classes to subclass object, which is needed
to give them properties (they weren't originally as there seems to be
some problem if a module's primary classes derive from object). Change
accessor methods of the same to Python properties, which are slightly
more convenient and "more Pythonic".

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23271 3bd1e089-480b-0410-8dfb-8563597acbee