Prevent scroll events during trackpad three finger swipe

[jackqu7]

At the start of a three finger swipe gesture on trackpad, a small scroll event is generated and not suppressed. This results in the active window scrolling slightly every time a three finger swipe gesture is used.

The variable magicMouseThreeFingerFlag seems to be preventing this behaviour for Magic Mouse, so introduce a similar flag for trackpad. This fixes the issue.

[jackqu7]

@aaronkollasch just wanted to bump this please, would be great to get it in the official build if possible

[aaronkollasch]

I should have some time after next week to review. Thanks for your patience!

[EugeneTarasenko]

I should have some time after next week to review. Thanks for your patience!

@aaronkollasch, It's been almost 5 months. Haven't you had some time to review this pull request yet?