Custom map names (backwards compat edition)

Hanicefrequested to merge
custom-map-names into next

Porting Lactozilla's Custom map names patch to Classic, with some slight code adjustments to Lua to remain backwards compatible with scripts that depend on 1100-1103 being special maps in G_ExitLevel and G_SetCustomExitVars.

Downstream patch: STJr/SRB2!2248 (closed)

Merge request reports