Add Tile Reference Properties #3235

From https://github.com/mapeditor/rs-tiled/issues/123#issuecomment-1012847157:

Tile references would be really useful as custom properties. Without them, you can either use GIDs or a custom formatted string, however, both approaches have their downsides:

• GIDs are meant to be an internal artefact and one should not rely on them being constant, plus they are only valid in a map context.
• Custom formatted strings require a custom parser and have no editor validation.

It would be really nice if there was a custom property type that allowed selecting a tile from a specific tileset (in a map context) or selecting a tile from the current tileset (in a tileset context). It could be then serialized as a GID or local tile ID which could be parsed by the library you are using.