winit-wayland: add hold gesture support

[jondkinney]

Summary

Adds the Wayland hold gesture (zwp_pointer_gesture_hold_v1) as a new
WindowEvent::HoldGesture { device_id, phase }.

A hold gesture fires when one or more fingers are placed on a touchpad without
significant movement. Per the protocol, its canonical use is to stop kinetic
("momentum") scrolling — there's currently no way for a winit app to observe a
stationary finger-rest on Wayland, since a rest produces no wl_pointer.axis
event.

This builds directly on the existing pinch-gesture support: it reuses the
already-bound zwp_pointer_gestures_v1 manager and the PointerGestureData
window tracking, adding only the hold object and its dispatch.

Details

• Manager bind raised from 1..=1 to 1..=3 so the hold gesture (added in
  manager v3) is available where the compositor supports it. Older compositors
  bind a lower version and keep pinch unchanged.
• The hold object is created only when the bound manager is v3+
  (manager.version() >= 3), so v1/v2 compositors are unaffected.
• Begin → TouchPhase::Started; End → Ended, or Cancelled when the
  compositor reports the gesture cancelled (e.g. fingers moved into a real
  scroll) — mirroring the pinch handling, including the subsurface filter.
• New WindowEvent::HoldGesture { device_id: Option<DeviceId>, phase: TouchPhase }
  in winit-core (no delta; hold carries none).

Platform support

• Wayland: implemented.
• Other platforms: unchanged (no equivalent stationary-hold gesture today).

Testing

• cargo build -p winit-wayland, full cargo build -p winit (x11 + wayland),
  and cargo clippy -p winit-wayland all clean.
• Runtime-verified on Hyprland (Wayland, zwp_pointer_gestures_v1 v3) with a
  small example logging window events: resting two fingers fires
  HoldGesture { phase: Started } then Ended on lift, or Cancelled when the
  rest turns into a scroll. The existing Pan/Pinch/Rotation gestures continue to
  fire alongside, unaffected.

[Murmele]

Same intention than in #4549