Restrict third-party shell plugin capabilities

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

shell/services/PluginBarStateApi.qml

@@ -0,0 +1,12 @@
+import QtQuick
+
+// Scalar-only view of the active bar for plugins that position independent
+// windows. The active Bar QObject is never retained here.
+QtObject {
+  required property string ownerPluginId
+
+  property bool barHidden: false
+  property int barSize: 0
+  property string fontFamily: ""
+  property string position: "top"
+}