Support external drop onto calendar

[GoogleCodeExporter]

Description of the new feature:
Support drop of arbitrary items onto the calendar.

Use Case:
Application has a list of contacts to one side of the calendar.
 Case 1: User drags a contact onto an empty block on the calendar.
         Application responds by creating a new Appointment.
 Case 2: User drags a contact onto an existing Appointment.
         Application responds by adding the contact to the Appointment
         and opening the appointment.

Assumptions:
1. Application code handles creating/updating appointments. gwt-cal only 
notifies that the drop occurred, either on a time block or an appointment.
2. I am assuming that gwt-dnd can handle separate drag/drop controllers for the 
same widgets, as I believe this will require this to avoid conflicting with the 
existing Appointment drag/drop operations.

Exceptions:
None known.

Code / psuedo-code:
To be provided. I will attempt to create and test an implementation, and add it 
as an attachment when done.

Original issue reported on code.google.com by [email protected] on 14 Jan 2013 at 4:54