postmarketOS power button dialog

Depends on #131 (closed).

How about we make the power button of each device work like this, with all UIs?

• press and release quickly: turn off/lock the screen
  • we would detect which UI is running, and then call something UI specific to turn the screen off (and lock it if the UI can do that)
  • we could also detect, if the lockscreen is running right now, and in that case only turn the screen on
• press and hold for a few seconds: display a postmarketOS specific menu

The menu would have these buttons:

• Shutdown
• Reboot
• Switch UI
• Settings (opens postmarketOS-settings: #135 (closed))