Improve scrolling behaviour and track current location

Submitted by Bryan W Clark

In bug 165120 Emil was cool enough to get us spacebar scrolling in Evince. However this is only the begining ;-)

We need to smooth out the scrolling action instead of snapping the view.

To help out with visual tracking of the scroll area we can overlay an image that gives a visual cue of what area of the page will be scrolled away.

Same can be done for the reverse.