Apply scroll factor by input device

[coleleavitt]

Summary

• Parse trackball { scroll-factor ... } like mouse and touchpad scroll factors.
• Choose scroll-factor settings from the libinput device kind before falling back to axis source.
• Treat continuous scroll source as touchpad-style fallback when the device kind is unknown.

Issues

• Fixes scroll-factor does not apply to trackball devices #2841
• Fixes scroll-factor does not affect on-button-down scrolling #2393
• Fixes scroll-factor silently ignored when tap is enabled in touchpad config #4202

Testing

• cargo test -q scroll_factor_falls_back_to_axis_source_without_device_kind
• cargo test -q scroll_factor_uses_libinput_device_kind_before_axis_source
• cargo test -q -p niri-config parse_scroll_factor_combined
• cargo check --message-format=short
• pre-push hook: cargo check