libxkbcommon: pull in updated recipe from Yocto master

clutter 1.14 needs libxkb 0.3
clutter-tweaks.bbclass: pull together bits shared by our various recipes

 * standard SRC_URI

 * customized FILESPATH: realname-PV:realname-major.minor:realname:files, where
   realname is the name of the upstream project rather than the BPN (i.e., cogl
   not cogl-1.0); this makes the patch directory names more self-explanatory,
   and managing point releases eaiser.
clutter: use clutter-tweaks class
cogl: use clutter-tweaks class
mx: use clutter-tweaks class
clutter-gst: recipes for 1.6