kcm/mouse: Add custom acceleration curve editor
GUI for libinput's custom pointer acceleration profile, building on kwin!6937.
Features:
- Bezier curve editor with 4 draggable control points
- Sensitivity Y axis (1.0 = physical match)
- Extension arrow for linear continuation beyond curve end
- Live input speed overlay (evdev, graceful degradation)
- Presets: Dynamic (default), Linear, Gentle, Aggressive with auto "Custom" detection
- Zoom controls for both axes
Depends on: kwin!6937