Reinstate helper_alternate_class() function

Following the discussion in www.mantisbt.org/bugs/view.php?id=17437, this restores the helper_alternate_class() function previously removed by dhx.

The purpose is to avoid errors with plugins relying on this function following the upgrade.

To facilitate plugin authors' efforts in updating their code to use CSS for table row colors alternation, the function now triggers a deprecated warning. 

Since displaying inline errors from within an html tag will mess with the output, a mechanism has been implemented to delay the printing of such errors, which are displayed in a block at the bottom of the page.