Touchscreen: Improve scrolling in absolute point mode.

Touchscreen: Improve scrolling in absolute point mode.

* Scrolling is now done by wiping over the screen. There's no acceleration or
  kinetic scrolling yet though. But it works rather well (previously
  you held the edges of the list to scroll).
* Improve scrollbar scrolling so that it keeps scrolling even if you leave the
  scrollbar area.
* Hide selection during scrolling
* Prevent accidental hitting of the list title (which means go back) during
  scrolling
* Don't go into context menu after scrolling when leaving the screen on an item

In general, it's very much like scrolling in native lists in Android,
except there's no kinetic scrolling and everything is still line based, but with
the feature that the scrollbar enables jumping around in the list and very fast
scrolling.

Thanks to Dustin Skoracki for his ideas and half of the code for this.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@28145 a1c6a512-1295-4272-9138-f99709370657