Wacom-gui needs minor update

Would be nice if you could update your driver to be compatible with 
- Mint cinnamon ("cinnamon-session" and remove the "only visible to mate" in the *.desktop file)
- PyQt5
- Python3

I tried to tweak your code, but this caused more errors and I didn't know how to correct them. In my opinion, you only need to tweak your code a bit so it works for Mint and higher dependencies too. 