Support TMX texture relative Source path

[DeathPhoenix22]

Current
The TilesetLoader is always looking for the Source texture path starting from the TMX map itself. This "forces" all textures to be placed next to the Maps & inside the levels folder. It basically consider only the Image name instead of the asset path.
New
The TilesetLoader will now take the complete path of the Source which allows us to place the resources anywhere relative to the file. For example, it's now possible to place the Tileset image file in the textures to avoid duplicating textures in both the textures folder and the levels folder. This also facilitate structure in the Levels folder (levels in different folders can share the same Source texture asset).
e.g. : <image source="../../textures/level/biome/ground.png"

[DeathPhoenix22]

I've spent some time trying to provide a test, but the loadImage method of TilesetLoader fails silently.

} catch (e: Exception) {
    log.warning("$imageName cannot be loaded using mapURL=$mapURL", e)

    resize(getDummyImage(), w, h)
}

Hopefully, the lack of automated testing is acceptable in this case?
Thanks,