Build plugin menu only when plugins change, and build the first version of it before it is required to ensure that the first processor box popup menu appears fairly quickly.

git-svn-id: svn://localhost/ardour2/branches/3.0@6092 d708f5d6-7413-0410-9779-e7cbd77b26cf

libs/ardour/ardour/plugin_manager.h

@@ -69,6 +69,9 @@ class PluginManager : public boost::noncopyable {
 	void set_status (ARDOUR::PluginType type, std::string unique_id, PluginStatusType status);
 	PluginStatusType get_status (const PluginInfoPtr&);
 
+	/** plugins were added to or removed from one of the PluginInfoLists */
+	sigc::signal<void> PluginListChanged;
+
   private:
 	struct PluginStatus {
 	    ARDOUR::PluginType type;

libs/ardour/plugin_manager.cc

@@ -156,6 +156,8 @@ PluginManager::refresh ()
 #ifdef HAVE_AUDIOUNITS
 	au_refresh ();
 #endif
+
+	PluginListChanged (); /* EMIT SIGNAL */
 }
 
 void