Simplify plugin dialog open with language entries

[Garry Yao]

It's now over-killed to open dialog in plugin, which has language files defined, e.g. the a11yhelp plugin:

	var langCode = editor.langCode;
	langCode =
		plugin.availableLangs[ langCode ] ? langCode :
		plugin.availableLangs[ langCode.replace( /-.*/, '' ) ] ? langCode.replace( /-.*/, '' ) :
		'en';

	CKEDITOR.scriptLoader.load( CKEDITOR.getUrl( plugin.path + 'dialogs/lang/' + langCode + '.js' ), function() {
		editor.lang.a11yhelp = plugin.langEntries[ langCode ];
		editor.openDialog( commandName );
	});

It would be as easy as just adding the available language list to the plugin meta, the language file loading should be handled be the dialog plugin instead.