Merge branch 'flatpak' into 'master'

Gitlab CI: add flatpak nightlies

Closes #29 and #15

See merge request !34
4 jobs for master in 32 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
failed #305792
flatpak

00:00:32

failed #304980
flatpak

00:03:01

canceled #304979
flatpak

00:00:01

failed #304545
flatpak

00:00:40

 
Name Stage Failure
failed
flatpak Test
Successfully extracted cache
$ flatpak-builder --stop-at=${FLATPAK_MODULE} app ${MANIFEST_PATH}
App dir 'app' is not empty. Please delete the existing contents or use --force-clean.
Uploading artifacts...
WARNING: dia-dev.flatpak: no matching files
WARNING: _build/meson-logs/meson-log.txt: no matching files
WARNING: _build/meson-logs/testlog.txt: no matching files
ERROR: No files to upload
ERROR: Job failed: exit code 1