QmlDesigner: Ensure item library content is visible

Make sure the ScrollView content area is visible after content size
changes.

Fixes: QDS-3795
Change-Id: I1f7408db9ad318a5c54b5186569dd04235a3d952
Reviewed-by: Mahmoud Badri <mahmoud.badri@qt.io>

share/qtcreator/qmldesigner/itemLibraryQmlSources/ItemsView.qml

@@ -87,6 +87,12 @@ ScrollView {
         itemContextMenu.close()
     }
 
+    onContentHeightChanged: {
+        var maxPosition = Math.max(contentHeight - height, 0)
+        if (contentY > maxPosition)
+            contentY = maxPosition
+    }
+
     Item {
         id: styleConstants
         property int textWidth: 58