Add check for pre-existing application directory

[freakboy3742]

If you have an existing macOS directory, and you run python setup.py macos, you get a messy error message about a directory already existing. The same is true for other platform directories.

It would be better to replace this with a check - "do you want to replace this project y/n", and if y, delete the existing directory before regenerating.

[eliasdorneles]

This was fixed in 686fd65