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

kcmkwin/kwindesktop/main.ui

@@ -92,6 +92,12 @@
             <property name="enabled">
              <bool>true</bool>
             </property>
+            <property name="minimum">
+             <number>1</number>
+            </property>
+            <property name="maximum">
+             <number>20</number>
+            </property>
            </widget>
           </item>
          </layout>