Handle Tiled world loading

[adrien-bon]

When loading large maps (let's say 256x256 tiles), the number of entities to handle becomes huge, which cause poor performances.

A solution to this would be to chunk the loading of such a large map and only spawn tiles entities for the subset that we actually use. Fortunatelly, Tiled have a feature called "worlds" which is basically that: a huge map that have been split in smaller ones.

From an implementation point of view, the .world file would be loaded by a dedicated AssetLoader. The Asset would store the path to the maps (.tmx file) as well as their positionning information (which is available in the .world file).

Upon spawning a world, we would then load the maps it contains but only spawn maps that are currently "in use". By moving the camera, we would change which maps are used or not, and spawn / despawn maps accordingly.

Dev branch for this feature = https://github.com/adrien-bon/bevy_ecs_tiled/tree/feat/world

(original idea from @JtotheThree)

[adrien-bon]

Discussion for adding .world file parsing directly in rs-tiled crate started here: mapeditor/rs-tiled#319