pointer-gestures: add hold gestures

Hold gestures merely denote "there are fingers on the touchpad but they are not moving". As touchpad touches are generally fully abstracted, a client cannot currently know when a user is interacting with the touchpad without moving - no motion events will be sent in this case.

The two use-cases here are:

• 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.

Minimum todo list:

• libinput implementation: libinput/libinput!602 (merged)
• mutter implementation: https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1830
• GTK implementation: https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/3454
• wlroots implementation: https://github.com/swaywm/wlroots/pull/3047
• KWin implementation: https://invent.kde.org/plasma/kwayland-server/-/merge_requests/273 and https://invent.kde.org/plasma/kwin/-/merge_requests/1226

cc @JoseExposito

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Reviews:

• @emersion

Acks:

• wlroots (@emersion)
• GTK/Mutter (@carlosg)
• KDE (@zzag)

changed title from WIP: pointer-gestures: correct description of pinch to WIP: pointer-gestures: add hold gestures

This is missing a get_hold_gesture request.

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• 7ffa8839 - pointer-gestures: add hold gestures

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mentioned in merge request libinput/libinput!602 (merged)

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• 8a977620 - pointer-gestures: add hold gestures

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resolved all threads

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• fd49f790 - pointer-gestures: add hold gestures

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@whot if you install both Mutter (https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1830) and GTK (https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/3454) merge requests you'd be able to stop kinetic scrolling using hold gestures.

I'd like to spend a couple of days testing before I remove the "WIP" tag, but so far it is working as expected.

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• fd49f790...177ff911 - 6 commits from branch wayland:main
• 9673a25e - pointer-gestures: add hold gestures

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Peter labeled a version (v1.18.900) including hold gestures.

It'd be nice if we could merge the protocol, or fix what's needed, so it's easier to continue with Mutter/GTK.

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