How to pair legacy Bluetooth devices in Linux Mint 18.1 Cinnamon and Xfce

For a long time, I have had a problem with my bluetooth Canon X Mark  1 calculator mouse. It simply would not connect to my bluetooth. It would recognize it; it could scan it and find it, but it would not pair. After many hours of research and a lot of experimentation, I have discovered a very simple solution.

My mouse is an older bluetooth device which uses a pin to pair. When the device does not give you a pairing pin to put in, the default pin is often 0000. I know that this is the case with my mouse, because that's how I would pair it on Linux Mint 16 (and it connects easily).