DED: Added 'Title image' MapInfo definition property

If defined the title image is used in place of the title text, when
this is displayed at the start of the map and during the intermission.
IdTech1Converter|MapInfoTranslator: Translate Map.titlepatch (MAPINFO) => Map Info.Title Image (DED)
All Games: Moved definition of map title images to MAPINFO

This gets rid of the last use case for episodeNumberFor() (which means
there is no longer any guesswork involved in determining the episode
for any given map).