Implement activation via xdg-activation

So far we've used it to pass around the activation token so phosh can track startup. Use it for actual activation too. This fixes raising windows to the top in docked mode when phosh uses with GTK >= 3.24.35.

This supersedes !393 (closed) hence /cc @dos

There's one TODO as we don't track the seat to activate the view in map() which is simple to fix as a follow up. Note that the protocol only talks about the seat the view got activated from though.

Note that this needs phosh!1233 (closed) as otherwise view activation will always fail as the splash has the focus.

I have so far only tested on recent GTK3 not older ones as in PureOS Byzantium but better to check.