test save/load with multiple elements

no code changes necessary, test passes as expected
save/load with connections

base.py

=> Connectors have their own title (fixed by type of Element), which is
stored in the DB to link connections. This means no more
"combine_title()" which might set the total title to None if the Element
did not have title.
=> create get_class_for_element_type() function, as a wrapper around the
dictionary access
=> loading elements from DB now uses the constructor instead of a
factory class method
=> add get_connector() to get the Connector object with a specific title
=> Connection::__init__() and set_flow() modified so the link the
between element_model and connection_model is saved OK (element_model
needs to be saved to DB first, before you can link a connection_model to
it).

flow.py
=> load_from_database() adapted with a second loop over all elements to
do connection-specific setup
=> get_element() works with element_id too
=> connect() should set the flow of the Connection object to save it to
the DB