graphics: Build level name graphics for FreeDM.

First stab at fixing #11. Generate DMWILV* lumps containing FreeDM
level names, and include these in the IWAD as the CWILV* lumps
as appropriate.
musics: Separate out configuration for FreeDM.

FreeDM's levels should have different music to Freedoom (#10). Look
for MIDI files named D_DM?? to use for the levels.

For now this unfortunately means that the FreeDM levels are currently
lacking any music, as they're now all using the dummy (empty) MIDI
file. But hopefully that can be remedied soon ...