gtkgesture: Handle hold gestures

gtkgesture: Handle hold gestures

Hold gestures are used to bring existing gestures on touchpad
semantically closer to touchscreen gestures.

Touchpad gestures observe hold gestures with a matching amount of
fingers and emit their begin and end signals when fingers are detected
or removed on/from the touchpad.

When a hold cancel event is detected, it is required to wait a few
milliseconds until the next event(s) are received to avoid emitting
multiple begin signals.

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