Automatically add new region map sections

[huderlem]

When creating a new map, if the user enters a region map section that does not already exist, append it to the existing region map constants. Its default display name can simply be the part after MAPSEC_, and remove any underscores, I guess.

Allow similar functionality in the region map editor when modifying the Layout tab.

I did some quick scanning over pokeemerald to see if this is problematic or not, and I didn't see any reason this shouldn't be supported.