Greatly reduce memory usage required by Konsole's scrollback buffer and improve releasing of scrollback memory to OS when no longer needed.

* Reduce amount of memory required for representing characters in scrollback buffer by not storing formatting data for each character.  Instead store UTF-16 characters and formatting ranges
* Allocate memory for scrollback buffer use a custom mmap-based memory pool - this allows the memory to be released to the OS sooner when freed.

There is a small performance cost when printing a large number of lines - see bug report for figures.

Patch by Michael Meier

REVIEW: http://reviewboard.kde.org/r/802/
BUG:176974


svn path=/trunk/KDE/kdebase/apps/konsole/; revision=987102