[WIP/RFC] add missing infolabels for better playlist data support in video GUI

This PR is aimed at adding missing infolabel support for videoplaylist handling in GUI. Currently all "videoplayer.next*" only work for PVR and videoplayer.offset(1).* is not possible at all (which is available for musicplayer).

This is WIP and RFC. Code is untested. Would like to get feedback if this is wanted in general or how to solve it more properly (best would be to unify how skins interact with players, regardless of video- or musicplayer).

ping @jmarshallnz, @ronie, @opdenkamp (for the labels initially added for PVR)