App closing

Allow to close app from the overview. Needs https://source.puri.sm/Librem5/phoc/merge_requests/39 merged in first.

This is not perfect since we e.g. have to close dialogs before their parents but will likely do for now. We'll do better after switchiing to xdg_toplevel_management.

A possibe 'optimization' would be to close on app_id only instead of matching the title as well.

closes: #67 (closed)