Fixed|F API: Custom status of contained files in M_ReadFileIntoString()

Presently the custom status of any contained file (e.g., lump in a WAD) is
not inherited from the container in FS1, so this case must be checked for
specifically.
Refactor|IdTech1Converter|MapInfoTranslator: Track custom status of MAPINFO data, split output

Partition the translated definitions into two data sets, according
to whether the source MAPINFO data is "custom" (from an add-on).
By separating the data we can continue to track the custom status
for individual definitions, should multiple MAPINFO data be merged
from multiple sources.
IdTech1Converter|MapInfoTranslator: Synthesize Music definitions from MAPINFO

Doomsday introduces a "song" abstraction to simplify the creation of
custom soundtracks and introduce more flexibility for mod authors.
Unfortunately, MAPINFO must assume a 1:1 relationship between maps
and their configured "song", i.e., music lump name and cd-track num.

Consequently in order to maintain mod compatibility we'll generate a
set of new Music definitions with an id naming convention that allows
mods using MAPINFO to work alongside mods using DED.

However, this means that if a mod uses MAPINFO - the new Music ids
make it incompatible with existing DED custom soundtrack mods. This
is a fundamental incompatibility cannot be solved easily.