Add submit callback and better validate callback for variables in form

While trying to add file variables, we've found the need for more form related variable callbacks.

- Add a new variable 'submit callback' that will be invoked like this on form submission:

  myvariable_form_callback($variable, $options, $form, &$form_state);

- Add more parameters to the existing 'validate callback', see variable_form_element_validate() in patch

  myvariable_validate_callback($variable, $options, $element, $form, &$form_state);

The rest of the patch are minor adjustments to make these callbacks work on several variable forms (included realm forms).