Use Layer::Top instead of Layer::Overlay

https://github.com/pop-os/cosmic-workspaces-epoch/issues/66. If we want
to hide the workspaces overlay when a popup is opened, that still needs
to be done, but using the same layer as the panel should be good in
general. There's no clear reason to have it on the `Overlay` layer.

src/main.rs

@@ -210,7 +210,7 @@ impl App {
             id,
             keyboard_interactivity: KeyboardInteractivity::Exclusive,
             namespace: "cosmic-workspace-overview".into(),
-            layer: Layer::Overlay,
+            layer: Layer::Top,
             size: Some((None, None)),
             output: IcedOutput::Output(output),
             anchor: Anchor::all(),