postmarketos-ui-cage: new package

For handheld console devices (even without touch support), you don't often have or want to use connected keyboard and some full-size UI. In my opinion, kiosk-mode WM will be best for them.

Use case may be to quickly start moonlight on boot, or use some controller-aware launcher. Not decided yet, so package itself is pretty barebone.

Needs CAGE_UI_COMMAND environment variable to tell what to run, and will fallback to postmarketos-demos if that is installed.