[Icons KCM] Give measure delegate a text

It appears at some point `ItemDelegate` changed to collapse
when it has no text, breaking the popup size calculation.


(cherry picked from commit 9e1ab30cca)

kcms/icons/package/contents/ui/IconSizePopup.qml

@@ -44,6 +44,8 @@ QtControls.Popup {
 
             QtControls.ItemDelegate { // purely for metrics...
                 id: measureDelegate
+                // without text it has no real height
+                text: "For metrics only"
                 visible: false
             }