CP-6093: The Window menu item on the top MainWindow menu should be disabled when
no forms are open. Since there is no event notifying about the Application.OpenForms
changing, created a new base class for all XenCenter dialogs and wizards with a
static event to which the main windw can subscribe.

Signed-off-by: Konstantina Chremmou <konstantina.chremmou@citrix.com>