Tap actions

This Pull Request covers the introduction of configurable tap actions.
The Double-Tap action and the Three-Finger action can be set to any of these values:

- Nothing
- Show original Artwork (= default behavior for Double-Tap and currently built into Muzei)
- Next Artwork (sends the Next command to the ArtSource)
- View Artwork details (opens the provided view Intent from the ArtSource)

This pull request has been re-created based on the newest master branch