Schedule time is not validated sufficiently

Problem/Motivation

- _workflow_transition_form_element_validate_time does not sufficiently validate the time. for instance, enthering characters, not numbers, does not geneare an error.
- the function may be moved into the new WorkflowTransitionTimestamp.php element;
- standard drupal validation rules may be applied.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes