Make the search message dialog header way prettier, like it is in KCMs

I think I've heard of this before...

src/qml/SearchPage.qml

@@ -26,24 +26,38 @@ Kirigami.ScrollablePage {
         room: root.currentRoom
     }
 
-    header: RowLayout {
+    header: QQC2.Control {
+        padding: Kirigami.Units.largeSpacing
+
+        background: Rectangle {
+            color: Kirigami.Theme.backgroundColor
+
+            Kirigami.Separator {
+                anchors {
+                    left: parent.left
+                    bottom: parent.bottom
+                    right: parent.right
+                }
+            }
+        }
+
+        contentItem: RowLayout {
+            spacing: Kirigami.Units.largeSpacing
+
             Kirigami.SearchField {
                 id: searchField
                 focus: true
-            Layout.topMargin: Kirigami.Units.smallSpacing
-            Layout.leftMargin: Kirigami.Units.smallSpacing
                 Layout.fillWidth: true
                 Keys.onEnterPressed: searchButton.clicked()
                 Keys.onReturnPressed: searchButton.clicked()
             }
             QQC2.Button {
                 id: searchButton
-            Layout.topMargin: Kirigami.Units.smallSpacing
-            Layout.rightMargin: Kirigami.Units.smallSpacing
                 onClicked: searchModel.search()
                 icon.name: "search"
             }
         }
+    }
 
     ListView {
         id: messageListView