Restrict third-party shell plugin capabilities

Reported-by: Roger Piñol <rogerpicar@gmail.com>

shell/plugins/panels/clock/Panel.qml

@@ -119,7 +119,9 @@ Panel {
   // Summoning by hotkey moves no pointer, so a hover the bar was still
   // holding must not keep the center indicators revealed behind the panel.
   function setCenterHoverRevealSuppressed(value) {
-    if (root.bar && "centerHoverRevealSuppressed" in root.bar)
+    if (root.bar && typeof root.bar.setCenterHoverRevealSuppressed === "function")
+      root.bar.setCenterHoverRevealSuppressed(value)
+    else if (root.bar && "centerHoverRevealSuppressed" in root.bar)
       root.bar.centerHoverRevealSuppressed = value
   }