Allow #element_validate to be a php callable

Problem/Motivation

It is possible to use callables (in my case array('ClassName', 'staticMethod')) for #element_validate in drupal core to define elements in hook_element_info(). This doesn't work with Webform though, as it checks it with is_function().

Proposed resolution

Just replace is_function with is_callable as was done in Drupal 7.50 in many places.

Remaining tasks

Please review if this makes sense.