Update workspace settings to be closer to mockup; use cosmic-config

Not quite like mockup. May need a custom widget to have radio buttons
that work the same way.

Also needs images used in mockup.

app/src/pages/mod.rs

@@ -21,6 +21,7 @@ pub enum Message {
     PanelApplet(desktop::panel::applets_inner::Message),
     DockApplet(desktop::dock::applets::Message),
     Appearance(desktop::appearance::Message),
+    DesktopWorkspaces(desktop::workspaces::Message),
     Input(input::Message),
     External { id: String, message: Vec<u8> },
     Page(Entity),