#4363 (ok 4.3) Plugin interface should be locale aware

[Ongun Kanat]

Last month I've reported bug: https://sourceforge.net/p/phpmyadmin/bugs/4326/ . In my daily usage of phpMyAdmin I've detected the all toupper and tolower operations in libraries/plugin_interface.lib.php are using standart strtoupper and strtolower. But they does not work in non-ASCII systems such as Turkish in my case. The mbstring plugin of PHP is very useful in this cases. It successfully converts strings to upper and lowercases. As a solution I reccomend usage of mb_strtoupper and mb_strtolower functions instead of strtoupper and strtolower.