Split exchangeEvent into ExchangeEvent and ExchangeBaseItem

Signed-off-by: Michel <info@1st-setup.nl>
Changed from the less readable, but maybe more protable, defineProperty to __proto__ inheritance

Signed-off-by: Michel <info@1st-setup.nl>
Added new ExchangeTodo object.
Needs some tweeking.

Signed-off-by: Michel <info@1st-setup.nl>
Got exchangeTodo object working.
Now need to create the right updateXML function for it.

Signed-off-by: Michel <info@1st-setup.nl>
Fixed small problem where the new status on an invitation was not preset to the right value.

Signed-off-by: Michel Verbraak <michel@1st-setup.nl>
Fixed problem where deleting an occurrence on an invitation failed to ask for a response to send back.
Also the occurrence was not removed from the view. Only after a restart.

Signed-off-by: Michel <info@1st-setup.nl>