gedit 3.8 compatibility

This updates the code to be compatible with gedit 3.8:

*   Updated .plugin file (with a separate .plugin file for gedit <= 3.6).
*   Fixed empty tab closing on launch. This should also be compatible with gedit <= 3.6, though I haven't tested it yet.

Would be great if this is merged soon as I have some other ideas I'd like to implement after this :-)