Build gnome-shell as a binary linked against libmutter-wm, instead of
a module to be loaded by libmutter-wm. Move the majority of
initialization-type stuff from gnome_shell_plugin_start() into main().
We still build libgnome-shell as a shared library, so that the linker
doesn't discard all the methods that are never called from C.
https://bugzilla.gnome.org/show_bug.cgi?id=641724
