Refactored tbl_structure.php by extracting mathods

Extracted methods PMA_getColumnsWithUniqueIndex() and PMA_getReservedWordColumnNameMessages() from "tbl_structure.php".
Refactored "Response.class.php" such that it can add multiple PMA_Messages at once.
Refactored "tbl_structure.php" by Creating "display_structure.lib.php" file to separate presentation layer from the logic layer.