In this file I'll try to explain which classes use what classes and why

Classes that need the application object:
-----------------------------------------

1 SignalEditor, because it needs access to the CommandManager of the app to add
  and remove signal handlers.

  Solution: pass the application object to the SignalEditor constructor.


2 Editor, because it is the one who create instances of  SignalEditor and it
  needs to pass the app instance to it (see 1). Same thing with EditorProperty
  (see 6). It also needs direct access to CommandManager to set the special
  name property

  Solution: pass the application object to the Editor constructor


3 ProjectView, because it needs access to the Commandmanager of the app to
  remove widgets (when the user press the 'delete' key

  Solution: pass the application object to the ProjectView constructor


4 Project, because it needs access to the CommandManager of the app to
  remove widgets (when the user select the delete option in the popup menu
  or press the 'delete' key when the widget in the interface is focused)

  Solution: pass the application object to the Project constructor


5 Placeholder, because when the user clicks on a placeholder usually a widget
  needs to be created, so it needs access to the CommandManager and also to
  the current selected palette button (add_class)

  Solution: pass the application object to the placeholder constructor


6 EditorProperty, because it needs to set the properties of the widgets and so
  it needs access to the CommandManager of the application instance

  Solution: pass the application object to the EditorProperty constructor

7 Popup, because from this menu the user can execute some commands and it
  need the CommandManager to do so.

  Solution: pass the application object to the Popup constructor

Note that for those objects of the same class that share the same app instance
we could use a class variable. The reason we are not using it is that we need
to initialize that variable in some place. If that place is main.py:launch we
are going to need to import all that classes in main.py and circular
dependencies probably arise