Copy tileset.txt, layering.json, and fallback.png to output

[Inzarcon]

The tileset.txt, layering.json, andfallback.png files are not copied to the output directory. The original compose.py does not seem to do this, either. One can usually expect them to already be present in the game files. However, they are still needed when composing a tileset to be published as a standalone or when making changes to them. As such, more explicit handling would be better. This means:

• Check if files already exist in output directory
• If they do, probably game files -> Validate if tileset.txt is the same.
• User options to always copy/overwrite the file(s), i.e. in case changes were made.
• Additional logger warnings and status messages depending on case.

[Inzarcon]

Basic file validation now exists for the source directory so the same principle can be used to check the output directory.

Moved to v0.1.3