Implement cursor hiding on scroll

This begins to address #662, by hiding the cursor when the page is
scrolled to prevent a distracting cursor and accidentally triggering
hover effects.
Use cursor: none and pointer-events: none to hide the cursor on scroll
Remove unused code in CursorHider
Use a boolean to track whether we are scrolling in CursorHider
Check if cursorHideStyle is in the document before remove()-ing it
Merge branch 'hideMouseOnLinux' of https://github.com/mrmr1993/vimium into mrmr1993-hideMouseOnLinux