[CR] git submodule setup

[int-ua]

Demo: https://github.com/CDDA-Tilesets/CDDA-Tilesets

Pros: ➕

• ability to make separate releases for each tileset independently;
• source code for releases will only contain relevant data;
• separate /issues for each tileset;
• flexibility in storing only desired tilesets history for all contributors;
• ability to manage virtually unlimited amount of tilesets in one org comfortably.

Cons: ➖

• a couple of additional steps in setting up for new contributors;
• additional time spent one time on moving all the existing commits into the new structure, with cleaning up unrelated history for each submodule https://gist.github.com/korya/9047870 git filter-repo loses track for the folder when it was moved, but I've found a solution: subdirectory-filter preserving renamed directory history use-case: only sequential commands work newren/git-filter-repo#182
• CI may need a minor update Use git submodules for tilesets, mods and languages CleverRaven/Cataclysm-DDA#28707 (comment)
• tilesets with symlinks to other tilesets will not detect changes automatically

Bonus: masters can be renamed to mains.

[int-ua]

Closing so it won't get in the way.