extracted methods from tbl_relation.php to tbl_relation.lib.php for rendering html
added methods PMA_sendHtmlForTableOrColumnDropdownList, PMA_sendHtmlForColumnDorpdownList, PMA_sendHtmlForTableDropdownList
added methods PMA_handleUpdateForDisplayField, PMA_getQueryForDisplayUpdate
Merge pull request #650 from scnakandala/refac_tbl_relation.php

Refactoring tbl_relation.php