Using Gtk.Notebook on Controller to switch between views.
* Added controller to main window, istead of using WeekView directly.
* Using current_view variable in CalendarPlugin so it is not necessary to mimic
  every public function from a view inside the controller.
* Managing connection to current_view signals (disconnect and reconnect whenever
  there is a change in the view).
* The two_weeks_view is not working properly since a Week object has seven days.
Changed Week to WeekSpan that shows N weeks instead of just one.
* Now two_weeks_view is working properly.
Reworked structure of Controller.
Setting combobox content dinamically from Controller views.
Changes in the status bar.
* Removed label that was used as intermediator.