Add categories or folders to organize large numbers of Tilesets #3220

I'm using Tiled with an infinite map, and am running into a organizational problem where sorting through my Tilesets using the dropdown arrow is quite tedious even at a relatively small scale (~30 Tilesets). Here's an example I found from a very similar forum request/question:

It would be nice if the dropdown was sorted into arbitrary "folders" or categories, for example one for each area of a infinite map, or for smaller games one for each purpose like "Roads", "Buildings", "Plants", etc.

Tileset categories could be configured in the Tileset itself as a new property, or in a new re-ordering user interface. If defined as a property making nested categories would likely not be possible, but that feels easier to accomplish.

I was planning to use the infinite map for my entire game, but to do that with the current dropdown I think I will have to prefix each Tileset, which will help with ordering but will still require scrolling through the list of Tilesets to find what I want.