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.
libdoomsday|DED: Track the custom status of DED definitions
Merge branch 'common-episode-definition' of github.com:skyjake/Doomsday-Engine into common-episode-definition

Conflicts:
doomsday/client/src/def_main.cpp
doomsday/plugins/idtech1converter/src/mapinfotranslator.cpp
libcommon|InputBindingWidget|Client: Forming 64-bit pointer to event symbolic name
DED|libdoomsday: Added if (not) custom condition to Mods directive

Presently supported with Map Info definitions only. This conditional
allows one to instruct the DED parser to apply the modification only
when the "custom" status of the definition matches.

For example, to reassign the music of E1M1 to a different song only
if the associated Map Info definition has not yet been modified by
an add-on:

Map Info mods "E1M1" if not custom { Music = "e1m1"; }