mutter: s/clutter-tweaks/clutter/
Update cogl/clutter/mx to use PACKAGECONFIG

This matches changes in patches sumbitted to oe-core.
mx: renamed to mx-1.0 and mx-2.0 to match oe-core
Fix clutter.bbclass rename

Move it under classes/, where it belongs.
clutter.bbclass: updated to match patches submitted to oe-core
Updated README to note branch naming conventions.
Remove glib and libxkbcommon recipes (not needed against oe-core master)