Fixed set_bg_once
Remove unneeded Util::restore_saved_bgs
Remove unused static setbg methods
WIP: finished refactor of setbg into base w overrides
Removed unused bits from restore refactor
Removed long set_bg overrides
Merge remote-tracking branch 'mbp/setter_refactor' into setter_refactor

Conflicts:
	src/SetBG.h
Fix compiler/runtime bugs
Determine which setter to use at runtime

This can be overridden by using --force-setter on the command line
to be one of "xwindows", "xinerama", "gnome".