Add the new API method connect( username, password ) to the interface
and implementations of a backing store schema and to the overlying
SchemaObj.

This will be used by the client-server GUI to construct a disconnected
backing store instance, which must later be connected through the GUI's
login screen in order for the user to do anything with the GUI.  Only
one connection per user is used, so it's a pretty limited and clumsy
GUI I have in mind.  It's more of an experiment than a serious attempt
at a general purpose user interface that I'd actually recommend anyone
to use.

But hey, it'll be *quickly produced*, so for some applications, that's
more important than a polished interface.  If it works, that's good enough
for one-of and short-term-usage scenarios.