It would appear that some platforms process the KILL_FOCUS event
after running TransferDataFromWindow(). This change makes sure
that the evaluation is done no matter the order.
Fixes: lp:1793911
* https://bugs.launchpad.net/kicad/+bug/1793911
Normal SetValue() call would temporarily change the displayed
value, but as soon as the text widget receives focus again, the original
expression (not evaluated) is restored.
To avoid this, the original expression is cleared in the associated
NumericEvaluator object.