Add continuous scrolling action

[Luwx]

KDE Plasma has a nice feature that simulates scrolling by pressing the wheel button

https://wayland.freedesktop.org/libinput/doc/latest/scrolling.html#on-button-scrolling

Apparently it sends "continuous" scroll events instead of wheel, so it feels very responsive like using a scrollbar.

The config could look like:

mouse:
  gestures:
    - type: swipe
      direction: any
      mouse_buttons: [ middle ]
      lock_pointer: true

      actions:
        - on: update
          input:
            - mouse: [ continuous_scroll_by_delta 0.5 ]

[pallaswept]

The tradeoff with the native way is that you lose the primary selection clipboard (middle-click paste) from the mouse; it would be really nice to be able to keep that, and trigger autoscroll with a gesture instead!