== ToDo ==
* Redo KCalcButton

* Use validator for constant value line edits in configure.

* Have a look at Evan's new stack engine kcalc_core.cpp. But be very careful
not to create new bugs. Also think why statistic-function in an independent file.

* Implement integer arithmetic for (unsigned int, signed int,
unsigned short,...). Look here at some of Evan's code.

* think what to do with cos, sin, etc. right now, as work-around cast KNumber to double,
use standard libc cos, sin, etc., cast back