[players] Make omxplayer be derived from dvdplayer, part 1

This pulls the cosmetic changes from https://github.com/xbmc/xbmc/pull/5084 into an easier to merge PR.
I believe this should result in no functional change for dvdplayer.
omxplayer changes a little by updating to latest dvdplayer.

The first 6 commits bring omxplayer much closer to dvdplayer.
The next 4 commits makes the PlayerAudio/PlayerVideo more consistent between dvdplayer and omxplayer, by removing unused functions, using const consistently and making methods protected.