Allow to tweak debug flags at runtime via DBus
This makes debugging more dynamic so one can enable things in an already running session. I opted for DBus rather than e.g. varlink as we have nice property mappings that makes this simple. We might want to switch at some point. Will be more useful with !621 (merged) but we can land it independently.
It might make sense to add this to mobile-settings in the compositor tab in an "Debug Options" dialog at some point, hence /cc @gautham_x