Create object factory CMake file, move vtk_add_override

This macro is copy/pasted to several files, more of the functionality
should be moved soon too.

Change-Id: If29eb8eec07508aa591758483be3dd189fe1cca8
Now generate the object factory files in a function

This moves the logic for object factory override generation to a
central function, and removes lots of copy/paste code in our CMake.
Also removed/retired the OpenGL2 object factory stomping variation
as it is no longer needed and could produce unwanted side effects.

Change-Id: I4a5ee2db76f73d57eded1d9a9240cefc4918cd74
Merge topic 'refactor-object-factory-cmake' into master

cc8d1471 Now generate the object factory files in a function
7def3bbc Create object factory CMake file, move vtk_add_override