They should be part of the backend-specific API.
The only backend that has an enumeration type is the X11 one, and it's
small, so we can simply put it there.
This is not an ABI change: the backend-specific symbols are still in
the same SO. You'll be required to import clutter-x11.h to have access
to the GType method at the source level, whereas before just importing
clutter.h would have sufficed. The only user of that enumeration was a
function declared in clutter-x11.h, anyway.
9d40fafa2a