Add opaque background to dock overlay window

The transparent dock overlay at LayerTop let maximized app content
bleed through, visible as a colored band (e.g. VLC's orange toolbar)
behind the favourites bar.

Add a Rectangle with Kirigami.Theme.backgroundColor using the Window
colorset, matching the pattern used by NavigationPanelComponent.

containments/homescreens/folio/qml/main.qml

@@ -176,6 +176,13 @@ ContainmentItem {
         LayerShell.Window.exclusionZone: -1
         LayerShell.Window.keyboardInteractivity: LayerShell.Window.KeyboardInteractivityOnDemand
 
+        Rectangle {
+            anchors.fill: parent
+            Kirigami.Theme.inherit: false
+            Kirigami.Theme.colorSet: Kirigami.Theme.Window
+            color: Kirigami.Theme.backgroundColor
+        }
+
         FavouritesBar {
             id: dockOverlayBar
             anchors.fill: parent