Basics for drag and drop support
Extended drag and drop support
* Now the task clicked is identified and a copy of it is moved around.
* Modified the rectangle/task coordinates to a cleaner look.
Using gradient instead of solid colors for drawing tasks
Edit the task being dragged when button is released.
* Now both the start and due dates are saved after a object is dragged and dropped.
* The task continues to have the same duration, though.
Removed any reference to end_day since it wasn't being used
* No more reference to self.view_end_day
* Now the date_generator function uses only @start and @numdays as parameter.
* Removed self.footer
Improved drag and drop, included support to enlarge/shrink tasks.
* Now the task being dragged itself is changed, instead of a copy.
Change the cursor when moving/hovering a task.
Fixed bug when task was deleted
* Removed reference to location where the task was drawn before
Deal with overflowing tasks (start/end in previous/next weeks)
* Creates a differentiation when drawing a task that continues in other days not
  shown in the present view (their rectangles have an 'arrow' now)
* Centralizes the task title considering only the space on the present view
* Every task now is drawn using a rounded rectangle
Fix bug where tasks were still been referenced in next/previous weeks even though they were not drawn
Keep a task selected in order to edit or delete it.
* Not editing/deleting a random task anymore.
* If task is clicked, it will keep the focus, unless user clicks on 'empty' space
Added horizontal scroll to go to next/previous weeks.
* Horizontal scroll only moves 1 day at a time.
* When clicking on the < and > buttons, still goes back/forward @numdays.
Goes to the beginning of the next/previous week when pressing < and > buttons.
Added a label to show current year.
Added color as a Task variable.
* Added random_color function to generate similar colors.
Fixed bug when drawing tasks overflowing to both sides.