Move all of the mutter code into a new libmutter-wm.so, split its
main() method into meta_get_option_context(), meta_init() and
meta_run(), add methods for using in-process plugins, and add
libmutter-wm.pc pointing to the new library.

The mutter binary is now just a tiny program that links against
libmutter-wm. The --version and --mutter-plugins options are handled
at the mutter level, not in libmutter-wm, and a few strange unused
command-line options (--no-force-fullscreen and --no-tab-popup) have
been removed.

https://bugzilla.gnome.org/show_bug.cgi?id=643959