SystemDialog: add separator below SSD titlebar of desktop version

This makes it looks consistent with other QtQuick windows where we do
the same thing to visually separate the titlebar from the window
content.

lookandfeel/contents/systemdialog/SystemDialog.qml

@@ -38,6 +38,14 @@ Item {
         window.show()
     }
 
+    Kirigami.Separator {
+        anchors {
+            top: parent.top
+            left: parent.left
+            right: parent.right
+        }
+    }
+
     ColumnLayout {
         id: column
         anchors.fill: parent