Use index-ring distance for trackpad pinch gesture

[aaronkollasch]

Scales the threshold to trigger three-finger pinch gestures according to the Index-Ring distance, as set in the Characters tab.

The default charRegIndexRingDistance is 0.33, and the original distance threshold was 0.15, so the distance multiplier should be roughly 0.455.

Fixes #34

[aaronkollasch]

It looks like there are two defaults for charRegIndexRingDistance:

• 0.33, set in createDefaultPlist
• 0.3, set in loadSettings2 if the charRegIndexRingDistance key is not found.

I am treating option 1 as the true default, as the charRegIndexRingDistance should be present in any plists derived from the default plist, which suggests that the multiplier should be 0.455 and not 0.5.

[aaronkollasch]

Perhaps in the future, if a Settings... dialog gets added to the Trackpad tab, I might add a slider for trackpadIndexRingDistance, but for now this is a much more minimal PR that still adds customizability. Will merge now.