Add more robust checks for non-existant mappings for GM objects/layers + add tileset mapping

- Moved "prepare" functions to root level to prevent these functions from being defined over and over again
- Add tileset mapping so you can have tilesets in LDtk with different names from their GM counterparts
- Parser will skip over objects it can't find in GM
- Parser will skip over layers it can't find in GM
- Parser will skip over tile layers if it can't find the appropriate GM tileset
- Parser will now create a tilemap on a GM layer and set the appropriate tileset (if found) if a tileset on the GM tile layer is not set (this allows for the tileset to be changed per level in LDtk, and not have to create a separate room for each level)
- Objects and layers/tilemaps now respect the "offset" property defined on layers in LDtk

Author: FaultyFunctions