Add hold gestures

Hold gestures are notifications about fingers on the touchpad. As touchpad touches are generally fully abstracted, a client cannot currently know when a user is interacting with the touchpad without moving.

Hold gestures are primarily designed for two interactions:

• hold-to-interact: where a hold gesture is active for some time a menu could pop up, or some object is selected, etc.
• hold-to-cancel: where e.g. kinetic scrolling is currently active, the start of a hold gesture can be used to stop the scroll

Since hold gestures by definition do not have movement, there is no need for an "update" stage in the gesture.

Status:

• libinput: Implement hold gestures https://gitlab.freedesktop.org/libinput/libinput/-/merge_requests/602
• Wayland protocols https://gitlab.freedesktop.org/wayland/wayland-protocols/-/merge_requests/81
• Mutter: This MR
• GTK: gtk!3454 (merged)