webui: do not show closed dialog
    
    Fixes issues when dialog is not removed from `IPA.opened_dialogs` registry when dialog.close() is called while the dialog is not shown, i.e., while other dialog is shown. Without it, the dialog is could be incorrectly displayed.
    
    New dialog's property `opened` handles whether dialog is intended to be opened.
    
    How to test:
    
    Add new host with IP address outside of managed reverse zones to get error 4304.
    
    https://fedorahosted.org/freeipa/ticket/4656
    
    Reviewed-By: Endi Sukma Dewata <edewata@redhat.com>