Selectable players

I've provided a runtime plugin preference for selecting the music player to probe for media information.

Currently available players are still xmms and amarok. I don't yet have an effective means of detecting where libDCOP.so and libDCOP.h are, so I've had to hardcode them in autoconf.ac for my system (sorry). I probably won't fix this, though, because my next step is to link to supporting libraries at runtime, so the compiler doesn't need to know where they are.