this.actor removal: Inherit all UI objects from Widgets or Actors

Together with !500, this removes all the this.actor usage in the gnome-shell code.

• Inherit all the objects from Clutter.Actor's or St.Widget's
• In some cases, introduced new Interfaces that are implemented by various classes (AppView)
• fixed some issues we had caused by accessing to "destroyed" classes after their finalization.
• Ported some other classes to inherit from GObject when we needed to pass them through signaling

I've split the MR in atomic commits, so in theory this could also be split in multiples MRs.