Use Tab to switch bar panels

shell/Ui/Panel.qml

@@ -27,6 +27,10 @@ Item {
     Qt.callLater(function() { popoutSwitchClosing = false })
   }
   function toggle() { opened ? close() : open() }
+  function switchPanel(direction) {
+    if (bar && typeof bar.switchPanelFrom === "function") return bar.switchPanelFrom(root, direction)
+    return false
+  }
 
   // Read a single value from this panel's inline shell.json entry, with a
   // fallback for missing/null values. Matches BarWidget.setting().