Simplify passing of events between activities

- Don't use event index in the event list as that can change if events are updated
  or added.
- Pass a LocalEvent instead of event ID in the intents when possible.
- Move event add/update functions to base EventListFragment.
Move removeEventFromList() to parent class
Improve handling of tab fragments

- Add constants to represent tab indexes.
- Add accessor functions for fragment contained in each tab.
Load newly accepted event from remote database