Console variable to select music preference

Adds musicpref to console and sound options menu, which allows users to select whether to prioritize MIDI or Digital music without disabling the other - in current SRB2 behavior, MIDI music will only play if Digital is disabled entirely, or not available for the given track.

S_GetMusicLumpNum, S_RecallMusic, S_ChangeMusicEx, GameMIDIMusic_OnChange and GameDigiMusic_OnChange updated to not assume digital music priority. The GamexxxxMusic_OnChange functions have been changed to have more consistent behavior between the two as well.

Small positive side effect of using P_RestoreMusic in the GamexxxxMusic_OnChange functions means that powerup music (speed shoes, invincibility) will restore in the correct position when reenabling the original MusicType they loaded in with.

Disabling digital music while in a map that doesn't have a MIDI track available, while MIDI is still enabled (or vice versa) will now print the standard "Music x could not be loaded..." error to the console, which deviates from current SRB2 behavior of simply stopping the music with no message - this could be changed, but I think the feedback can be helpful, and is consistent with receiving the error message in a similar situation on map load, which is currently the case.