Add Tile Reference Properties

[aleokdev]

From mapeditor/rs-tiled#123 (comment):

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.

[bjorn]

The changes done to add the "object" property should be helpful as a reference when implementing this: #2712

Maybe something similar to the ObjectRefDialog will be necessary here, but using a tileset view instead, probably with an additional dropdown to select the tileset to reference. Alternatively, the implementation will need to integrate with the existing tileset view in both the map editor and the tileset editor, to allow selecting the tile from there.