Add control reference to web plugins

Add reference to the Control instance in BaseWebPlugin for web plugin
code to use in a similar manner to the instance reference in instance
plugin code.

packages/web_ui/src/util/websocket.jsx

@@ -46,6 +46,7 @@ export class Control extends libLink.Link {
 		 */
 		this.plugins = plugins;
 		for (let plugin of plugins.values()) {
+			plugin.control = this;
 			libPlugin.attachPluginMessages(this, plugin);
 		}