lookingGlass: Add switches for debug flags
This adds a new "Flags" tab to looking glass where several debug flags
can be toggled at run time. This includes:
ClutterDebugFlags
ClutterDrawDebugFlags
MetaDebugPaintFlags
MetaDebugTopics
MetaContext::unsafe-mode
Fixes: #3968 (closed)
Depends on mutter!2028 (merged)