Add missing lower and upper bounds to the rows spinbox.
Summary:
0 rows makes no sense.
Setting than 20 rows make no sense when the number of desktops
is limited to 20.
BUG:387594
Reviewers: Plasma, graesslin
Subscribers: kwin, plasma-devel, KWin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D9199