[ical4j 4.x] Update event handlers part2

[sunkup]

Update handlers part2 + tests:

• AvailabilityHandler(),
• StatusHandler(),

// scheduling

• SequenceHandler(),
• OrganizerHandler(),
• AttendeesHandler(),

// extended properties

• CategoriesHandler(),
• UnknownPropertiesHandler(),
• UrlHandler(),

// sub-components

• RemindersHandler(accountName)