window: Add a "back" gesture

This makes moving back to the contacts list more comfortable on a phone.
2 jobs for librem5-back-gesture in 5 minutes and 55 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build Flatpak
passed #233612
flatpak
flatpak:master

00:05:53

 
  Build Arm Flatpak
failed #233613
ARM64
flatpak-arm64:master

00:00:02

 
Name Stage Failure
failed
flatpak-arm64:master Build Arm Flatpak
Running with gitlab-runner 12.4.0 (12.4.0)
on arm02 zx28gbiB
Using Docker executor with image jsparber/ci-arm64:arm64 ...
Pulling docker image gitlab-runner-helper:12.4.0 ...
ERROR: Job failed: Error response from daemon: pull access denied for gitlab-runner-helper, repository does not exist or may require 'docker login': denied: requested access to the resource is denied (executor_docker.go:186:1s)