Launch splash support

This adds initial splash support for launching apps via gtk_shell1 wayland protocol. We show a full screen splash in mobile mode and currently nothing in docked mode (we could show a smaller splash there). One can swipe the splash screen away in case something gets stuck.

It needs

• phoc!259 (merged) to work at all
• For DBus activated apps we need GNOME/glib!2227 (merged) (or a similar solution) - without it we just just don't show a splash for those apps
• For X applications at least https://github.com/swaywm/wlroots/pull/3124 which works for most but not e.g. libreoffice where we need to expand on protocol support (see below)
• Tests

Future additions

• support xdg-activation (phoc#223 (closed))
• expand x startup support (https://github.com/swaywm/wlroots/issues/2968)
• report startup failures (needs design @bertob) (see #602)
• reduce flickering (e.g. by holding on for the surface a split second longer or until we see a toplevel with the right app-id, we have code for that already)

I think we could initially merge once the wayland phoc/phosh protocol is sorted out and design is halfway sane. I've dropped some bits that also track app-id's on toplevels, etc but the intention is to extend the app-tracking object with that in the future.

Closes: #183 (closed)