Pma string

After recent OOPing of the string functions they had to be accessed in three different ways, some using $GLOBALS['PMA_String'], some using $GLOBALS['PMA_StringType'] and others using traditional PMA_STR_ prefixed functions. In this pull request I've brought them all under $GLOBALS['PMA_String']. Ofcourse some of the names used for the new interfaces, classes can be improved, feel free to suggest.