[RFC] Subtitle cycle

This is a RFC for http://forum.xbmc.org/showthread.php?tid=183437 so to get a bit more eyes on the namings etc.

Some considerations
1) This is very quickly thrown together and I will test it more before final PR
2) This PR does not include keymaps being changed (as they should most likely have CycleSubtitle)
3) Should we perhaps quick return when only one subtitle is available.

As an extention to 3) we should perhaps add so skins can hide the "Next subtitle" button when its only one available?