Add DurationSpinBox + delgate and use in taskeditor.
This works as a double spinbox with the addition of changable units.
It's meant as a replacement for the (somewhat complicated) DurationWidget.

Caveats:
* Based on QDoubleSpinBox, should possibly be KDoubleSpinBox (KLocale vs QLocale).
* Precision is always 2.
* Rounding may change the value when the unit is changed. This shouldn't be a big problem as the *normal* use case is to select the unit, then enter a value.