Reestructured signal handling between widgets.
Removed packing of Header and AllDayTasks from main window to WeekView.
Fixed bug on size of Header when AllDayTasks scrollbar is/isn't visible.
* WeekView now handles vadjustment change events in order to notify
  the header when the scrollbar will be present or not.
PEP8fied code.
Changed selected_task to store task_id instead of task object.
Allow creation of a task using DnD.
* Happens whenever user starts DnD in an empty area of the widget.
* Highlight cells during drag and drop where new task will be created.
* Dialog is opened at end so user can edit title/dates, or even cancel creation of task.
* Fixed small bug on requester when deleting a task.