wayland-outputs: Refactor event sending to ensure we're consistent

This makes us behave the same both on bind and when an output
changes. In particular, we were not sending scale and done events on
output changes. We were also unconditionally sending mode events on
output changes even though these should only be sent if there is an
actual mode change.

https://bugzilla.gnome.org/show_bug.cgi?id=766528