[Feature Request] Allow Porymap to create new gMapGroups if the one specified doesn't exist by default

[LOuroboros]

Sup.

Earlier, I decided to organize the map groups that I assigned to some old maps I made in a pet project of mine.

I put some of those maps correctly in gMapGroup_TownsAndCities, but some other maps I put them incorrectly inside of gMapGroup_IndoorRoute124, so I decided to fix that.

I had no problems whatsoever doing that, and it's not even something within the scope of Porymap, but it's relevant context to the rest of this ticket's contents.

Now, as it turns out, what I found out when I wanted to make a new map using Porymap, is that the program is incapable of creating new gMapGroups, which means that I'll have to be constantly tweaking those as I create more maps for that pet project of mine.

Thus, I'd like to see Porymap automatically creating a new gMapGroup if the label that the user specified at the time of making a new map does not exist by default 👀