Add support for metadata in tilemaps (DEF-2089)

It would be really useful to be able to add metadata to the tilemap. This could be information about spawn points or environmental effects that impact game logic. Maybe waypoints for navigation. The external tool Tiled has support for this (and theres a Tiled to Defold exporter).

All of this was discussed in an e-mail thread sent to defold@king.com. The gist of it is this:

Tadic: "To clarify, what I was talking about was adding support for Tiled (and possibly some more popular format, as in the attached table), which would also take the underlying data (control data per tile). This would require some design and additional implementation but is hardly rocket science and would have a lot of bang for the buck for users."

Ritzl: "It would be really awesome to be able to attach additional data to certain tiles in the tilemap to control spawn points for mobs, waypoints for navigation, sound triggers etc. All of it can be achieved by adding game objects containing this kind of data on top of the tilemap, but it's also very convenient to have this information in the tilemap itself."

And point 1.8 in this forum thread: (LINK REMOVED)