Revision 1620

Add dialogs and commands to open database connections and add
database layers.

* Thuban/UI/mainwindow.py (MainWindow.DatabaseManagement): New
method to open the database connection management dialog
(MainWindow.AddDBLayer): New method to add a layer from a database
(_has_dbconnections): New helper function to use for sensitivity
(database_management command, layer_add_db command): New commands
that call the above new methods.
(main_menu): Add the new commands to the menu.

* Thuban/Model/postgisdb.py (PostGISConnection.__init__)
(PostGISConnection.connect): Establish the actual connection in a
separate method and call it in __init__. This makes it easier to
override the behavior in test cases
(PostGISConnection.BriefDescription): New method to return a brief
description for use in dialogs.

* test/test_postgis_db.py (NonConnection): DB connection that
doesn't actually connect
(TestBriefDescription): New class with tests for the new
BriefDescription method