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>