Create a snap package for version 2.2

[pachulo]

This was tested by myself and the snap seems to be quite functional. But:

• Some interfaces need to be pluged manually: https://forum.snapcraft.io/t/snaping-musescore/704

Based off: https://github.com/pachulo/musescore-snap

This version could go to the beta channel of the store:

• https://snapcraft.io/docs/core/store

And this service could be used to create the snaps for the different archs and upload them to the store:

• https://build.snapcraft.io/

[lasconic]

Could you squash the commits? http://gitready.com/advanced/2009/02/10/squashing-commits-with-rebase.html
Do you have enough access to put build.snapcraft.io in place so we can have a build push to the store for every commit?