Summary:
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.

Test Plan:
Opened KCM (in English) set counter to 1 pixel and 2 pixels.
No longer had a big warning. Also appropriate number of s's appeared.

Reviewers: #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D3222