command: make window-scale property observable

Add a generic mechanism to the VO to relay "extra" events from VO to
player. Use it to notify the core of window resizes, which in turn will
be used to mark all affected properties ("window-scale" in this case) as
changed.

(I refrained from hacking this as internal command into input_ctx, or to
poll the state change, etc. - but in the end, maybe it would be best to
actually pass the client API context directly to the places where events
can happen.)
command: add window-minimized property (X11 only)

More or less requested by #1237.

Should be simple to extend this to other backends.