"Dev" versions of .mod files

[IhateTrains]

It would be good if we could keep both the Github and Steam WtWSMS versions in the launcher at the same time and have them easy to distinguish. My proposal:

• keep the current .mod files in case someone only uses the development version
• create copies of the current mod files (i.e. dev_WTWSMS.mod, dev_WTWSMS_AfricanPortraits.mod) with different names in the launcher (add [Dev] prefix or something similar)
• no folders have to be edited