wayland: Don't broadcast selection owner changes

wayland: Don't broadcast selection owner changes

When receiving a selection or when a drag icon enter a window, it was
targeted at a specific window. Lets emit the GDK_OWNER_CHANGE event
only for this window, instead of broadcasting.

Broadcasting has some nasty side effects. For example, if there was n
GdkWindows, and one would for every "owner-change" signal handler
receive n signals about the owner being changed.

An example of where this went a bit out of hand was gnome-terminal,
which added one listener per terminal window. This meant that if
one had m number of terminal windows, each time any one would loose or
gain keyboard focus, O(m^2) owner-change events would be emitted.

https://bugzilla.gnome.org/show_bug.cgi?id=754158