Add display options for wall-mounted tablet operation

[schup011]

• New options for screensaver in global settings (in addition to the existing ones) to keep screen on as long as music is played. This can be useful if the app is running on a wall-mounted tablet where somebody wants to see always what is currently playing.
  • Display clock when not playing
  • Display black screen when not playing, let then android system determine when the screen is completely off / locked.
• New option in global settings to always switch to NowPlaying view after an item has been chosen in ListView. This also supports wall-mounted operation: User wants to choose some music and then walk away.

Please have a special look on the BlackScreenSaver class, I am not completely sure whether the approach to make the screen black is robust, also against future Android versions (tested successfully, but only on Android 10).