Restrict third-party shell plugin capabilities

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

shell/services/PluginBarWidgetRegistryApi.qml

@@ -0,0 +1,24 @@
+import QtQuick
+
+// Detached widget-catalogue snapshot for third-party full-bar implementations.
+// Plugins can render the referenced components, but mutating this local view
+// cannot replace a registration in the host registry.
+QtObject {
+  id: api
+
+  property var widgets: ({})
+  property int revision: 0
+
+  function metadataFor(id) {
+    var entry = widgets[String(id || "")]
+    return entry ? entry.metadata : null
+  }
+
+  function availableIds() {
+    return Object.keys(widgets)
+  }
+
+  function has(id) {
+    return widgets[String(id || "")] !== undefined
+  }
+}