MDL-5583 mod_data: Add required fields

This is implemented for most fields but not all. The reason is that there are
some fields for which this does not make sense because no entry is a valid entry.

The supported fields are:
    checkbox
    file
    latlong
    menu
    number
    picture
    radio
    text
    textarea
    url

The unsupported fields are:
    date
    multimenu

mod/data/backup/moodle2/backup_data_stepslib.php

@@ -50,7 +50,7 @@ class backup_data_activity_structure_step extends backup_activity_structure_step
         $fields = new backup_nested_element('fields');
 
         $field = new backup_nested_element('field', array('id'), array(
-            'type', 'name', 'description', 'param1', 'param2',
+            'type', 'name', 'description', 'required', 'param1', 'param2',
             'param3', 'param4', 'param5', 'param6',
             'param7', 'param8', 'param9', 'param10'));