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.
Towards a fix of #716
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.