front: Added custom input mapping profiles.

User can save and load their own customized input maps. The maps are
persisted to files, located in a subdirectory of the user's game save
directory. Thus they persist through uninstalls and preference resets,
and being text files they are easy to manually edit and share.
front: String tweaks.

After playing for awhile, I think seeing "Save" and "Load" as the first
word improves recognition.