Ability to create folders within 'collection of images' tileset

Is your feature request related to a problem? Please describe.
For the purposes of my game, I have 3 tilesets with a bit south of 700 tiles divided in between them. It's hard to find a tile in there, and slows down the process of map making. Ideally it'd be easier for me to optimize the game with only one giant tileset.

Describe the solution you'd like
Now that we can reorder the tiles within a tileset while retaining the gids, I wonder if it would be possible to group them in folders within a tileset.
It would not affect the gid, and in the saved tileset there would still be the 'tiles' array with all tiles unchanged, but there would be an additional element, 'folders' for example, which would be an object/dictionnary with the structure of the folder hierarchy within the tileset.
The export option is not necessary for me in the game, just a way for tiled to remember the folders, there are probably more efficient ways to do that.

The issue names no files, tests, or entry points. Start by tracing how collection-of-images tilesets and their tile arrays are stored and displayed, then define how folder hierarchy should be persisted without changing GIDs; the work is done when folders can be created and retained in the tileset while the existing tiles remain unchanged.