Refactor: Moved command "listmaps" into ResourceSystem

Now uses Id1MapRecognizer to find all the recognizable maps from the
central LumpIndex rather than assuming a game-specific map id format
(i.e., MAPXX or EXMY). Naturally this means that map ranges cannot
be found without either a format hint from the game or using a more
complex algorithm (finding common prefixes in map ids, etc...).

The main purpose of this refactoring is to test Id1MapRecognizer for
locating all the maps from all loaded files ahead of building a map
collection in ResourceSystem.

Todo for later: Improve formatting of the map listing. (Once such a
map collection has been built generating a pretty listing should be
somewhat simpler).