Build: clean up

Moves snap & flatpak to build-aux

Adds tests to validate the appdata, desktop file & gschema, they are run by Gitlab CI for each build.

1/3 validate-desktop                        OK       0.02 s 
2/3 validate-appdata                        OK       0.07 s 
3/3 validate-gschema                        OK       0.01 s 

Ok:                    3
Expected Fail:         0
Fail:                  0
Unexpected Pass:       0
Skipped:               0
Timeout:               0
Clean up

cc @ZanderBrown