Support for multi-dimentional arrays in admin_config_set.php

This commit adds support for multi-dimentional arrays, as well as
correct handling of commas and '=>' within strings.

This is based on work by jspezeski; the original code was modified to
align with MantisBT coding guidelines, simplification of the recursive
function and fixing a couple of errors in regex.

Fixes #13298
WIP-Revised process_complex_value function

Use new regex
Code complete, needs to be tested
TODO: revise regex