Fixed drag and drop:

Fixed drag and drop:
- it's now possible to drag and drop several activities without the application crashing after the first commit
- it's now also possible to drag activities from a day to the parked activities
- and it's possible to drag activities from one day to another
Started changing the add new acitivity dialog:
- the idea is to use this dialog for adding new activities and for editing existing ones
- therefore an activity has to be passed as argument (if it's null -> add new activity, otherwise edit the given activity)