VstPlugin: added hasEditor() method

Added inline method hasEditor() with returns true or false depending on
whether the class received a window ID from the remote plugin.

plugins/vst_base/vst_plugin.h

@@ -47,6 +47,11 @@ public:
 
 	virtual bool processMessage( const message & _m );
 
+	inline bool hasEditor( void ) const
+	{
+		return m_pluginWindowID != 0;
+	}
+
 	void showEditor( QWidget * _parent = NULL );
 	void hideEditor( void );