Returns the valid types of plugins that can be used.
array An array of plugin type strings.
public static function getPluginTypes() {
return array(
'access',
'area',
'argument',
'argument_default',
'argument_validator',
'cache',
'display_extender',
'display',
'exposed_form',
'field',
'filter',
'join',
'pager',
'query',
'relationship',
'row',
'sort',
'style',
'wizard',
);
}