We want to be able to detect which dialogs are important and need to be
converted to async while not completely disallowing them. Allow only
first instance of such dialogs being Execute()d and warn when another
such instance tries to Execute().

Change-Id: I6742784fa95d9e3f9ff87ece294126d390ae9e9e
Reviewed-on: https://gerrit.libreoffice.org/50095Reviewed-by: Jan Holesovsky <kendy@collabora.com>
Tested-by: Jan Holesovsky <kendy@collabora.com>