Scroll usability improvements

* [x] Increase base scrolling speed on Linux (and maybe Windows) -- try to match Chromium's scroll rate of how many vertical lines of text it scrolls by with one mouse wheel click
* [x] Add PageUp/PageDown keybindings for scrolling
* [ ] Add tap-to-scroll to the scroll bar (See https://github.com/fyne-io/fyne/pull/5349)