use recurrence id when saving an event
Signal event change after occurrence change has happened

Avoid warnings in case property is binding to both properties.
[nemo-qml-plugin-calendar] Provide occurrence exception state information

When creating an exception to recurring event, there might be need
to know the recurrence id of the exception.
This commit provides the value by returning a change object, which
signals finishing and provides uid+recurrence id.
Add recurrence id to warnings