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20130804 (Sunday, 04 August 2013)
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Application code becomes easier. Instead of writing::

    insert_layout = dd.FormLayout("""
    event_type:25 type:25
    subject 
    project company
    """,window_size=(50,'auto'))
    
You can now write::    
    
    insert_layout = """
    event_type:25 type:25
    subject 
    project company
    """
    
Because `insert_layout` are now always initiated with that window_size 
when specified as a string.


I need to add application-specific rules for selecting the 
type of a :class:`welfare.notes.Note` in :ref:`welfare`.
To prepare this, I used the new app inheritance feature
and added a new module :mod:`lino_welfare.modlib.notes`
which extends :mod:`lino.modlib.notes`.

This was the first time that I extended a `detail_layout`, 
and it caused some subtle problems.

First I had to understand we *must* modify the original 
:class:`lino.modlib.notes.models.Notes`
class object by reassigning 
it's `detail_layout` and `insert_layout` attributes.
Subclassing the Notes class might look more clear 
coding style::

    class Notes(Notes):
        
        detail_layout = NoteDetail()
        
        insert_layout = """
        event_type:25 type:25
        subject 
        project company
        """

But that would fail because the original module also defines 
subclasses of Notes, and we want also all subclasses to use our 
new detail_layout or insert_layout.

The above trick still works only with the lates version because one 
thing was to replace::

  dl = getattr(cls,'detail_layout',None) 
  
by::

  dl = cls.__dict__.get('detail_layout',None)
  
in :meth:`lino.core.actors.Actor.class_init`.


But that wasn't enough. The above change revealed another problem.

Actors without their own detail_layout will use the layout 
defined on their parent. But if this parent is overridden 
in another app, then they must install themselves as the 
datasource for this detail_layout.

E.g. the `Persons` table is defined in 
:mod:`lino.modlib.contacts`
and overridden in
:mod:`lino_welfare.modlib.contacts`.
The welfare version does not override 
the `insert_layout`.