Snap: Fix audio and Files not found

This PR fixes two issues:

Audio-playback

No audio on animation timeline. This is fixed by adding audio-playback interface. This is auto-connected by default.

Users not finding their files

Can't browse or save files beyond the current folder @kamathraghavendra also indicated this is a common issue.

This is partly fixed by the recent grant of auto-connecting removable-media to krita.

This PR is the second part of the fix: it includes common directories in the "shortcuts" sidebar of the Qt File chooser.

This is how the file chooser looked before this PR:

This is how the file chooser looks now:

Although this solves the issue by making it clear for users where to find their files, this is more of a "workaround" than an actual fix. Ideally, Krita should use a native file chooser dialog. This would properly show usb sticks, connected smartphones and camera's, directories the user favorited etc. I'm still working on fixing everything that's required in order to get that working properly.