Implement finger-friendly "far-scrolling"

[snowwlex]

from http://old.fbreader.org/mantis/view.php?id=137

With fbreader on Neo Freerunner, it is currently hard to do "far-scrolling"
(that means, more than several pages up or down).

Although there is a scrollbar near window bottom, it is too thin to be used
with a finger, and even with a stylus tapping on that scrollbar gives
accuracy of +/- 50 screen pages :(.

I think it would be good to extend current "tap scrolling" as follows.
If user just taps at bottom part of the screen, then do page down - as now.
If user puts finger on bottom part of the screen and does not remove it,
the text should scroll forward at increasing speed.
Similar for top part of screen and scrolling backward.