[kcmkwin/desktop] Disable the OSD duration spinbox if the corresponding checkbox is unchecked

Summary:
In the old version of the KCM, one could change value of the spinbox
only if "Desktop Switch On-Screen Display" is checked.

Test Plan:
Before:
{F6503161}

After:
{F6503163}

Reviewers: #kwin, #vdg, graesslin

Reviewed By: #kwin, graesslin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D17754

kcmkwin/kwindesktop/package/contents/ui/main.qml

@@ -216,6 +216,8 @@ ScrollViewKCM {
                 QtControls.SpinBox {
                     id: osdDuration
 
+                    enabled: osdEnabled.checked
+
                     from: 0
                     to: 10000
                     stepSize: 100