Project Manager should correct invalid paths

[Ryder17z]

Example scenario

On windows, a slash in the name of a folder is for example not allowed, this could be easily solved by having the script responsible for creating the folder replace the invalid characters by a dash (-) or an underscore (_) for example.

[Calinou]

Feel free to open a pull request for this 🙂

The responsible code is somewhere around https://github.com/godotengine/godot/blob/a7d0e18a317085068c43be29bca1d280d03423a2/editor/project_manager.cpp#L805.