Add "Open Data Folder" to File menu

This PR adds an extra button to the File menu - "Open Data Folder." It just opens the folder where computer files are saved for easier access.

Windows: Fully tested, fused and non-fused - works fine
OSX: Fully tested, fused and non-fused - works fine
Linux: Fully tested, fused and non-fused - works fine

Edit: Tested more on OSX

Edit 2: Fully tested on all three platforms now.