This rewrites the wl_seat protocol implementation to adhere to the new
design principles.
Effectively, we've been supporting wl_seat v7 so the version was also
bumped from 5 to 7.
The main reason why we have factory methods is that up to some point,
kwayland had its own signal to indicate when globals have to be removed.
Now that all globals add destroy listeners for the wl_display object,
we don't have that signal. Most factory methods are equivalent to doing
new T(display).
Besides adding unnecessary boilerplate code, another reason to get rid
of the factory methods is to reduce the amount of merge conflicts. If
several persons work on implementing wayland protocols at the same time,
sooner or later someone will have to resolve merge conflicts in Display.
libwayland-server allows the wl_display accept client connections on
more than one socket. We currently don't listen on multiple sockets,
but it would be nice if Display supported such operation mode.
If we track the commit counts at compositor global, this will fail
horribly for anything other than the first text-input-v3 client, as for
new client the serial count will not be what it expects in the done()
request and it will simply consider events as outdated and will refuse
to accept those events
