Add categories or folders to organize large numbers of Tilesets

Nobody has claimed this yet.

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](https://discourse.mapeditor.org/t/tileset-categories/2530):


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.

Start at the Tileset dropdown in Tiled's editor and trace how Tilesets are listed and selected. Define how categories would be configured and displayed, including whether nesting is supported; done means large Tileset collections can be organized into categories without relying on name prefixes.

Written by the indexing model from the issue text.