backend: Allow setting a list of windowing backends
backend: Allow setting a list of windowing backends

Like CLUTTER_DRIVER, we want to allow users to specify a list of
backends to test, and fall back to the internally defined priority as a
default.

This requires changing the way the allowed backend string is parsed,
both for the CLUTTER_BACKEND environment variable and for the
clutter_set_windowing_backend() function. Existing callers are still
supported with the exact same semantics.

Distrotech/clutter,Distrotech/clutter,GNOME/clutter,GNOME/clutter,GNOME/clutter,Distrotech/clutter,GNOME/clutter,Distrotech/clutter,Distrotech/clutter,Distrotech/clutter,GNOME/clutter