Scroll factor configuration for scroll-method: "on-button-down"

[ProfLander]

Implements input.button-scroll-factor using the existing ScrollFactor type, and applies it to events of type AxisSource::Continuous.

I've followed the hyprland pattern of having the input -level setting govern button-style scroll speed so as to avoid per-device clutter or collision with existing scroll-factor settings, and added a clarifying button- prefix to ensure users don't look at it and assume a global modifier, rather than an event-specific one.

Closes #2841, #2393

[YaLTeR]

Maybe it should just follow the normal scroll-factor somehow? In sway there doesn't seem to be a separate button scroll factor. Besides, with specific input device config, there will be just a device-specific scroll-factor too.

[ProfLander]

There is no 'normal' scroll factor as-is; input.mouse.scroll-factor modifies AxisSource::Wheel events, and input.trackpad.scroll-factor modifies AxisSource::Finger events. Trackball and trackpoint have no hardware-specific equivalent, so are assumed to emulate via on-button-down, which emits AxisSource::Continuous.

Since those are semantically distinct, and all types of pointer can use button scrolling, they need their own sensitivity modifiers to avoid coupling. For example: A mouse's line-based wheel sensitivity shouldn't be tied to its sensor-based button-scroll sensitivity, in the case where both are active.

Hyprland's solution is to treat button scrolling as a singleton like I have here, but that does miss the idea that button scrolling is DPI-relative; more exotic setups with multiple pointing devices could benefit from button-scroll-sensitivity being a device -level setting instead of a global one.

[YaLTeR]

Hm well I don't see another way before some bigger refactoring so let's go with this. Do you mind adding it to the wiki page, not forgetting the Since: next release annotation?