Record origin position of scrollbars to improve grab mechanics

[ScoreUnder]

Test scrollbars with:

open Nottui

module W = Nottui_widgets

let () =
  W.string "Hello world"
  |> Ui.resize ~w:200 ~h:100
  |> Lwd.pure
  |> W.scrollbox
  |> W.h_pane @@ Lwd.pure Ui.empty
  |> W.v_pane @@ Lwd.pure Ui.empty
  |> Ui_loop.run

[c-cube]

I'm not going to comment on the code, but the behavior on this little test is nice. Maybe there could be a directory of small examples, with this as scrollbox.exe?

Also, unrelated, but the colors are bad, it's nearly impossible to see the cursor with dark grey on light gray. I suggest:

let scrollbar_bg = Notty.A.gray 15
let scrollbar_fg = Notty.A.lightblue

[let-def]

The behavior would be nice to have, but I am not fond of using permanent_sensor for this use-case.
It will essentially force the renderer to visit sub-trees and update every scrollbar if they move, while we are only interested in their movement if grabbing is on.

In practice, scrolling a list of scroll bars (ok, that does not happen every day) changes from being O(1) to O(n) (wrt to the number of scroll bars).

It would be more elegant to switch the sensor on only when grabbing the control.
I will write a patch later, and maybe rethink the sensor/grabbing mechanism.