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/lib/plugin.js

@@ -514,6 +514,13 @@ class BaseWebPlugin {
 		 */
 		this.info = info;
 
+		/**
+		 * Control link to the master server, not available until the
+		 * connect event in onMasterConnectionEvent is signaled.
+		 * @type {?module:lib/link.Link}
+		 */
+		this.control = null;
+
 		/**
 		 * Logger for this plugin
 		 *

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);
 		}