Custom tile layout for TileSet

[KoBeWi]

Godot version:
3.1 beta8

Issue description:
Right now when you use Tilemap, tiles are complete mess, because they are simply sorted alphabetically and you need to name them properly and set the tiles dock to proper size, so their order makes sense.

What I'm proposing is ability to select your own layout for tiles in tileset. I mean instead of keeping all tiles in a simple list, we should be able to make a tile palette where each tile has it's position (ofc not necessarily the same as in texture). Something like this:

which is created from such spritesheet:

So you create a tile normally, but also place it somewhere on TileSet's grid, so when you make Tilemap it doesn't display some alphabetically sorted list, but the actual grid (which might need horizontal scrolling then). This could be done either as addition to TileSet resource or as a new inheriting resource, because I guess not everyone will need this.

[KoBeWi]

Closing in favor of godotengine/godot-proposals#896