Link events for dragging/resizing by a separate property

Hey Adam,

I needed the possibility to group events by a separate property instead of their `id` for dragging and resizing. The scenario is a series of recurring events with overriden instances. Take a daily event over 5 days as an example. When taking one instance out of the series (like moving it one hour earlier) it should not be moved when dragging the whole series. So I added a new property to control which events are still in the group.

I could manipulate the `id` instead, but I'm using the real `UID` and don't want to hassle around extra handling there.

Cheers
Matthias