Clean up scrolling code a bit

No need to use prev/nextSlideTop, etc.
Fix Home/End and Scroll#deactivate
WIP Support dragging the scrollbar
Add separate file with long text to test scrolling
Throttle momentum scrolling by preventing wheel events

If time delta between two wheel events is not short or long enough,
prevent wheel events to prohibit momentum scrolling from scrolling
several slides at once.
Fix minor glitches (occasional scroll loop bug)
Clean up scroll callback
Handle window resizing
Support scrolling by dragging the scrollbar
Fix indentation
Restore previous position on refresh

For some reason Firefox always goes to the beginning after refreshing...