VLCPlaybackService: Fix index computation & refactor repeat current item mode
This merge request fixes the next & previous index computation that is now set to the current playing media index when it has not been initialized yet.
This avoids to reset the index to 0, which is the default value, therefore losing the current position in the play queue.
Closes #1434 (closed)
This merge request also brings changes to the repeat mode allowing to repeat the current item, that is now switched to the repeat all items mode when the next or previous button is pressed and the next or previous media in the play queue starts playing.
The current item mode behaviour keeps looping the current media if the user does not interact with the next/previous buttons.
Closes #1432 (closed)