QSpinBox can't handle plural suffixes. Something previously done by
KIntSpinBox.
Using setSuffix(ki18np("pixel", "pixels")).toString() does nothing, as
at the time of conversion we don't know which one to use.
This patch uses KPluralHandlingSpinBox and correct ki18np.
Note, "new" dependency was already linked implicitly in other kwin, but we need to add it for this KCM.