Re-enable skip previous/next as chapter jumps, when outside of playlist context

This is a follow up to #2809. 
This PR now re-enables chapter jumps using the skip previous-next action in case a video is not played back as part of a playlist.

The benefit is "playlist priority over chapters":

A) when playing a movie in a playlist, the behaviour is like #2809. User knows that a playlist is playing, so expects "skip next" to go to the next item in the playlist.
B) when playing a movie NOT in a playlist, the behaviour is like before. User may not even know or use the concept of playlists, and just wants to skip to the next (or previous) chapter. 

Note: From a code standpoint, the only comment I see is that DVD/OMXplayer is consulting g_playlistPlayer. Another way to achieve a cleaner separation could be to introduce a virtual method in the IPlayerCallback interface.