connection-editor: hold GApplication while the import dialog is running
This is done from new callbacks of nm_connection_list_create() and nm_connection_list_add(). In order for this to work, import_vpn_from_file_cb() had to be fixed to always invoke its callback. In turn, vpn_connection_from_file() had to be changed to pass its error result up to its callers instead of presenting a potentially redundant error message itself.
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