Widget for editing an integer field, with a calculator
Special use cases of the IntegerEditor :
we have a required integer field without a default.
so the model will do set_value( None )
since this is a required field, the user should be able to enter a value, 0 is a legitimate value.
when get_value is called, 0 should be returned if the user has set the editor to 0, and None if the user didn’t touch the editor.
so the editor should make a visual difference between None and 0, so the user can see he didn’t enter something yet
we have a non required integer field without a default
the model will do set_value( None )
the get_value() should return None and not 0. because in case it returns 0, 0 will be written to the db, causing an unneeded update of the db.