[players] Don't hardcode dvdplayer/paplayer

Currently playercorefactory wrongly maps various stream types to dvdplayer or paplayer.
It should be using videodefaultplayer/audiodefaultplayer.

Because of this we have pretended omxplayer was dvdplayer to avoid unwantedly using dvdplayer in these cases.

But, (optionally) using dvdplayer can be useful on the Pi. We can actually play sd (up to 640x480 MPEG-4 video) video in real time. This is useful for codec variants like DivX3, msmpeg and sorenson spark which we don't currently play with omxplayer.

Unfortunately there are many video add-ons that explicitly request dvdplayer, so there is still a hack to map that to omxplayer on Pi.

These patches are used in Gotham openelec on Pi, and appear to do the right thing.