clients: Add xdg-shell app_ids to sample clients
Make our sample clients set an xdg-shell app_id for their windows.
To the extent that we know what these clients are for, an example of best practice is mostly it. Make them do that.
@emersion I've extracted this from @mvlad's !611 (merged). I think this should be good for 10.0 as it's low risk and a definite improvement. Do you think it's worth landing?