KGlobalShortcutTrigger: Add support for line shape gestures

Follow-up to !96, building on prior commits of that branch.

Also known as mouse gestures, stroke actions, and probably a few other terms. I'm calling it line shapes now because "mouse gestures" is too mouse-centric (no reason we can't have these also for other pointing devices) and "stroke" is apparently too reminiscent of a serious medical condition.

The serialized format looks like (0.5;0)(0.5;1)(1;1)(1;-1) with no spaces anywhere. Example in kglobalshortcutsrc:

[kwin][$Triggers][Cycle Overview]
LineShape|Current=(0;1)(0;0)
TouchpadSwipe|Default=3:Up
TouchscreenSwipe|Default=3:Up

[kwin][$Triggers][Cycle Overview Opposite]
LineShape|Current=(0;0)(0;1)
TouchpadSwipe|Default=3:Down
TouchscreenSwipe|Default=3:Down