RFE: Advanced Thumb Detection

This issue is for tracking ongoing work on overall enhancement to thumb detection on clickpads. Design goals:

• Do not break existing functionality
• Maintain or improve gesture recognition accuracy (#13 (closed), #9, #5 (closed))
• Improve consistency of clickfinger and multitouch tap behavior (#10 (closed))
• Use context, plus hardware (pressure/size) when available
• Degrade gracefully on limited hardware
• Allow a thumb to rest anywhere on the touchpad (#4 (closed))
• Allow a thumb to rest before or after other touches
• Accurately detect scrolls and swipes with physical clickpad button down
• Avoid timeouts when possible (same order of actions -> same behavior, regardless of timing)

The idea is to draw heavily on macOS for inspiration, but without following its conventions blindly. Detailed analysis of macOS touchpad behavior: https://gist.github.com/mdmayfield/7720a0cd1e8b84a61e1543f801dc8245

• Prepare gesture recognition to work with thumb detection
• Refine specific size-based HW thumb detection (#97 (closed)) (to integrate later with larger design)
• Consider time-based instead of event-based speed threshold thumb detection
• Draw state machine diagram of proposed thumb detection
• Implement thumb detection state machine
• Find volunteers with various hardware to test and give feedback