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