[#2845] Replace fractional mouse_scroll_steps with per-element scroll speed calculations

[oznogon]

Remove mouse_scroll_steps member from scrolling elements, which defaulted scrolling to increments of 1/25 of the total height. For elements with very small overflows, this made scrolling very slow.

For scrolling text elements, replace that with 3 * text_size (3 lines/increment). For listboxes, replace it with the button height (1 button/increment).

Fixes part of #2845 (slow scroll speeds).