Add scrolling support to NumSlider

[Ampflower]

This PR aims to add scrolling support to NumSlider.

I am only opening this as a draft for now for some input as this isn't complete.

• Supports Chromium, Firefox, Epiphany
  • Note, some specific hackery was added for Epiphany.
    Someone with a MacOS machine would need to validate the following for Safari:
    • Shift + scrolling up correctly follows normally scrolling up, and vice-versa.
  • Some hackery was added for Firefox to tell it apart from a touchpad, as its deltaY isn't consistent between scrolling up & down.
• Has specific touchpad support
  • Has a way to adjust sensitivity
  • This includes detecting one to allow for fine adjustments with the scroll gesture.
• Has a way to reverse scrolling direction
  • This is necessary for natural scrolling. Safari on MacOS has a way to detect this, however no other browser and OS combination can. The best option is to give a way to flip the direction for this.

It is self-resetting to allow a device switch without feeling horrendous, unless you happen to switch to a mouse without leaving the slider.

I shall note that it does make a change to the backing data structure to coerce to an integer via |0, which I realise may break the modeller (although surprisingly doesn't?) and plugins expecting to allow non-integer input.

This does also introduce a check for bare WebKit specifically for Epiphany, as it is just detected as bare Electron otherwise.

[Ampflower]

Notable that scrolling fast does cause it to switch into thinking that you're using a touchpad; that is something to filter out.