Add music file[s] from within app

[jeremypw]

Towards a fix of #716

• Adds an actionbar/button linked to an open file[s] action.
• Open file[s] action shows a filechooser dialog for selecting multiple files
• Selected files are added to the playlist.

This PR does not implement the open folder element of the issue as that would require a separate action/button. That can be simply added later if this PR is accepted.