* Make both the Screen and the ScreenWindow keep track of lines scrolled.
The Screen class records lines scrolled in response to output from the terminal program. The ScreenWindow class records lines scrolled in response to the user dragging the scrollbar up and down.

This allows the scrolling optimisation to work for normal shell usage and for interactive programs such as man/vim etc.
* Avoid redrawing scrollbar if the value or range has not changed since the previous update.

* Add debug facility to count lines repainted on each update.
* Remove references to the 'history cursor' from the emulation.
* Some style tidy-up in TerminalDisplay::updateImage()