Add support for thumb button (Logitech MX Master) [enhancement]

[fallende24]

Description

All logitech mx master mice come with a button for your thumb (the one you press down/towards the desk, not towards the side), which doesn't seem to be supported at the time, as I only see space to configure buttons 4 and 5. When pressed within the area of Mac Mouse Fix, it doesn't do anything

Rationale

Would help add more functionality and use a button that's already there. Personally I'd use it to invoke the cross to select an area and take screenshots to the clipboard for example (instead of using cmd 4 and holding control which is a bit cumbersome)

Additional info

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[tejxv]

+1

[Hidlsh]

+1 - Using MX Master 3s