Xpra: Ticket #1034: X11 bindings refactoring

Follow up from #907, related to #640, #41, #990, #846, #881, #911

Issues:

• group-leader code calls gtk
• prop_get is called from an XSync context we manage, should never call into GTK - it doesn't, but we should make certain it never will
• split prop_get into X11_prop_get and inject X11 dependency?
• "can-focus" is not used
• "call_setup()" is inconsistent
• WM_STATE still a bit of a mess..
• iconic: boolean is not good enough? (need to map Withdrawn.. old clients can't do it)
• use managed connect for "notify::" in callers?
• simplify X11 signals: rename them X11zzz, ie: do_X11ClientMessage(..)
• XChangeWindowAttributes can be used to change OR!?
• x11_get_server_time -> replace with native X11 call?
• get_default_window_icon?
• _NET_WM_ACTION_CLOSE
• client_reparented should be original parent (cleaner?) - not safe
• WM_CLIENT_MACHINE: use XGetWMClientMachine
• add_event_receiver / remove_event_receiver: use WeakHashMap? to workaround gtk3 lack of set_data, would need route_event anyway - meh