Improve handling of overdue activities

Currently CiviCRM has a loosly-defined concept of "overdue" activities. Activity statuses are configurable and can be ambiguous as to whether the activity is completed or not. This improvement:

• Adds a class to each activity status to indicate whether it represents a completed or incomplete activity.
• Exposes this to the UI when configuring activity statuses.
• Adds helper functions for determining whether an activity is overdue.
• Adds a pseudo-field to the activity api "is_overdue" which allows sorting and filtering.