Detailed Description
This guy is a formula editor--kind of like a line editor but
you can do formulas. If you construct it with restricted
set to true, it prevents you from typing things it doesn't know
how to evaluate (like matrices, greek, integrals, etc.)
If you want to use your own evaluation, but want to have
the typing restrictions, you can use uglyForm() on a restricted formula
to get a c-style expression and setExtraChars() to allow characters
beyond the default allowed in restricted mode.
Features of KFormulaEdit include infinite undo/redo, cut/copy/paste,
and you can enter almost anything KFormula can display (I think).
To save a formula, use text(), save the returned string, and when
loading use setText(QString) with the saved string.
Keyboard shortcuts are availiable, but it's probably easier to
use a KFormulaToolBar (which see).
By default this is turned off for performance reasons.
If turned on, the signal sizeHint is emitted everytime
the formula gets a new size.