[C2] Add "duration" type extra field

[ywarnier]

Add a new extra field type duration that is expressed in seconds and shows in hh:ii:ss format.

More and more requirements have appeared recently to be able to give an estimate study/completion time for any item. Adding the availabillity of such a field type would make it easier to theme those times everywhere necessary.

This would probably be a new extra_field.value_type (FIELD_TYPE_DURATIONin extra_field.lib.php) of 28.

[christianbeeznest]

It is added with this PR #6238

Thanks for confirmation.