Only changing place of function.
Improvements on calculating offsets when dragging tasks.
* There is a bug in 'expand_right'.
Fixed bug when expanding_right a multi-row task in month view.
* Needed to update offset_y in this special case as well.
Cleared dnd_stop in month view -> not modifying task anymore.
* It is not necessary to change task dates when Dnd stops
  since it was already changed when moving task around.
* FIXME: change week_view as well.
Fixed bug with dragging tasks beyond the window size.
* Task was being drawn at end of month (row index was -1).
* FIXME: allow multi-row tasks to go to previous month when dragging.
Modularized invertion of cells when dragging from end to start.
Fixed bug when dragging to create new task.
* Highlighting cells wasn't working properly in some cases.
Removed calculation of offset from dnd_start to its own function.
Minor changes that don't affect anything.
Drawing longer tasks at the top of each row.
* Ordering tasks by desc duration.
* Small tasks can still be at the top if they fit, but only
  after the longer ones have been added first.
Aux methods to help deal with when there's more tasks than space.
Added method to get label height in all_day_tasks class.
Started working on hiding tasks that don't fit on cell.
* It is not working yet, these are just some preliminary changes to
  help the future implementation.
Trying to fix DnD of tasks. Not working yet.
FIXED bug with highlighting cells.
* Now both faded cells and the ones highlighted only on DnD to
  create new task are working properly.
Fixed bug on highlighting today cell.
* If today was at the last week of its current month and also
  appeared in at the beginning of next month, the wrong day was
  being highlighted.
Added class DayCell to show tasks that don't fit month view cell.
* DayCell is a Dialog that only draw the tasks, nothing more.
* FIXME: The calling of the dialog is not working properly yet,
  since we need a label or a button in the MonthView so we can
    click on to display the dialog for that day.
More work on hiding overflowing tasks. Not finished yet.
* Created Gtk.Label with link and callback function to display
  popup when clicked.
* The popup itself is working fine.
* FIXME:
  - Label not being displayed on the screen, even though it works
    properly when simulating clicking on it.
  - Not hiding tasks yet, they are still being drawn.
Working on hiding tasks. Still has bugs.
* The tasks are now being hidden, but we need to create a label to
  a cell that didn't need it before, but will need it after a
  neighbor had the task hidden (and the task spans to this cell).
Hiding tasks and showing clickable Label (but in the wrong place).
* Fixed bugs:
  - Label not being displayed on the screen
  - Not hiding tasks yet
* FIXME:
  - Label not overlapping with DrawingArea, but it is being added
    at the end of the month, in another container.