Support using external images for entities.

[HamdyElzanqali]

Could you allow us to select an external image (e.g. ".png" file that isn't part of any tileset used in LDtk) and use it for entities?
The only way possible right now to use a sprite for an entity is to add the sprite to a tileset, then select the tile from the tileset.

It seems like additional unnecessary work to add every sprite in the game to a single big tileset while I could've just selected the image. The benefit of this is to make the workflow more appealing and to visualize the level before actually running it.

Here is an example of the level when using only entities in LDtk.

vs the actual level in the game.