* Avoid allocating a new buffer to hold character data on every screen update.
  Instead a single buffer is created and maintained by ScreenWindow.
* Re-write Screen::getCookedImage() and rename to getImage() to make it possible
  to retrieve an image of a section of the screen which is not the same size as
  the terminal screen.
* Make various non-mutating Screen methods const


svn path=/trunk/KDE/kdebase/apps/konsole/; revision=749864