Scroll usability improvements #506
Description
- 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
- Add PageUp/PageDown keybindings for scrolling
- Add tap-to-scroll to the scroll bar (See Add tap-to-scroll to the scroll bars fyne-io/fyne#5349)