Add a touchpad scroll speed control in Mouse & Touchpad settings (libinput removed Scroll Factor)

[ja-plus]

Problem

The DDE Control Center "Mouse & Touchpad → Touchpad" panel only exposes pointer
speed (motion-acceleration), scroll direction, tap-to-click and scroll method —
there is no scroll-speed control at all. Many users (especially on the libinput
backend) find two-finger scrolling far too fast with no way to slow it down from the
GUI, and are forced to edit system config by hand.

Newer libinput has removed the libinput Scroll Factor property, so the old
"set a multiplier" approach no longer works. The only remaining knob is
libinput Scrolling Pixel Distance, which is capped (e.g. max 50 on a Huawei
ELAN962C touchpad) and gets reset on every reboot when applied via xinput set-prop.

Environment

• Distribution: Deepin 23 (DDE)
• Session: X11
• libinput via xserver-xorg-input-libinput 1.4.0
• Touchpad: ELAN962C:00 04F3:3109 Touchpad (i2c-hid)
• xinput list-props shows: no Scroll Factor; Scrolling Pixel Distance defaults to 15 with an effective max around 50

What exists today

• The panel already has a pointer-speed slider wired to motion-acceleration in
  com.deepin.dde.peripherals.touchpad, so the "slider → DConfig → backend writes
  libinput" path already exists.
• All that's missing is a scroll-speed key and its matching slider.

Proposed solution

1. Add a "Scroll speed" slider (Slow ↔ Fast) to the Touchpad settings page.
2. Add a new DConfig key (e.g. scroll-distance or scroll-speed) to the
   com.deepin.dde.peripherals.touchpad schema.
3. Do the actual libinput write in dde-daemon / SessionManager (same layer as
   the existing natural-scroll and scroll-method handling), rather than calling
   xinput from the control panel — so it works consistently on both X11 and Wayland,
   and can be swapped to a proper libinput API later.
4. Backend mapping: for now map the slider to libinput Scrolling Pixel Distance, and
   define the slider range from that property's valid bounds so we never expose
   unreachable values.

Workarounds users resort to

• Writing xinput set-prop ... "libinput Scrolling Pixel Distance" 50 into ~/.xprofile.
• Downsides: X11-only, needs a startup script to survive reboot, capped value, not a GUI.

Willing to help

I'm happy to open a PR covering the panel + schema + daemon write. I'd just like to
confirm maintainers agree with the "implement at the dde-daemon layer" direction before
I start, to avoid rework.