fix: compatible with the events calendar

[sapayth]

WPUF's event calendar post form wasn't working for version 6.x.x

new hook introduced
Filter hook: wpuf_tib_event_meta
Description: Opportunity to change The Event Calendar metadata just before WPUF is saving it to DB
@param array $event_data The event metadata
@param int $post_id The post id, in other words, The Event

Action hook: wpuf_tib_after_saving_event_meta
Description: Hook fired just after WPUF saved The Event Calender metadata to the DB
@param int $post_id The post id, in other words, The Event

Some test scenarios

1. Check backward compatibility for The events calender version less than 6
2. Test the hooks mentioned above. Here is an example of how to use hooks in WordPress.
3. Check without the WPUF Pro version

related PR in pro #512
fix #505