app: Offer restart on update

Flatpak gives us a clearly defined way of detecting when the app has
been updated, as well as a portal for spawning the latest version.
Use that to offer a seamless restart whenever the app is updated.

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4 jobs for wip/fmuellner/flatpak-updates in 8 minutes and 22 seconds (queued for 1 second)
latest
Status Name Job ID Coverage
  Source Check
passed eslint #160913

00:00:28

 
  Build
passed flatpak #160914

00:06:50

 
  Review
passed review #160915

00:01:03

manual stop_review #160916
allowed to fail manual