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 (actually a playlist but only one single item), 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.