Ensure make distcheck actually passes (and make clean)

Originally we generated all compiled resources within the data/ build,
which in fact broke the dependency chain within autotools due to relative
paths.
With this commit, each directory (build stage) now builds its own resources
which has the effect of ensuring make distcheck passes, and make clean. This
should ease package maintainence for other distributions somewhat.
budgie-plugin: Ensure comments are consistent with API
session: Ensure all files are included so distcheck doesn't bomb
Merge remote-tracking branch 'origin/cleanup'

Conflicts:
	panel/Makefile.am