Each keyboard focus change ends up calling the MetaWaylandDataDevice
counterpart, we don't need though to notify the current selection
again. In order to fix this, keep track of the current client, and
only emit the relevant signals when the focus switches to another
client.
The situations where wl_data_device.selection were emitted during
focus changes between surfaces of the same client was inocuous most
of the times, although it's prone to inducing confusing behavior
on context menu clipboard actions, as the closing menu triggers a
focus change, which triggers a whole new wl_data_offer being created
and given on wl_data_device.selection, at a time where there's already
ongoing requests on the previous data offer.
https://bugzilla.gnome.org/show_bug.cgi?id=754357
b18542f2b6