Port implicit default content items to use contentItem: property

See also
https://invent.kde.org/frameworks/kirigami/-/merge_requests/1338
and
https://invent.kde.org/frameworks/kirigami/-/merge_requests/1340

src/kcms/rules/ui/RuleItemDelegate.qml

@@ -21,7 +21,7 @@ Kirigami.AbstractListItem {
     highlighted: false
     hoverEnabled: false
 
-    RowLayout {
+    contentItem: RowLayout {
 
         Kirigami.Icon {
             id: itemIcon

src/kcms/rules/ui/RulesEditor.qml

@@ -197,7 +197,7 @@ KCM.ScrollViewKCM {
                 highlighted: false
                 width: ListView.view.width
 
-                RowLayout {
+                contentItem: RowLayout {
                     Kirigami.Icon {
                         source: model.icon
                         Layout.preferredHeight: Kirigami.Units.iconSizes.smallMedium

src/kcms/rules/ui/main.qml

@@ -143,7 +143,7 @@ KCM.ScrollViewKCM {
         Kirigami.SwipeListItem {
             id: ruleBookItem
 
-            RowLayout {
+            contentItem: RowLayout {
                 Kirigami.ListItemDragHandle {
                     visible: !exportInfo.visible
                     listItem: ruleBookItem