Create new config doesn't work

[SweBarre]

When launching the application without a ~/.mqttclient.yaml present the program crashes and throws the following error:

Traceback (most recent call last):
File "./mqttclient", line 358, in
CONFIG = read_config(CONFIG_FILE)
File "./mqttclient", line 313, in read_config
Gtk.DIALOG_DESTROY_WITH_PARENT,
File "/usr/lib64/python3.6/site-packages/gi/overrides/init.py", line 39, in getattr
return getattr(self._introspection_module, name)
File "/usr/lib64/python3.6/site-packages/gi/module.py", line 137, in getattr
self.name, name))
AttributeError: 'gi.repository.Gtk' object has no attribute 'DIALOG_DESTROY_WITH_PARENT'

A workaround is to manually copy the default_mqttclient.yaml to ~/.mqttclient.yaml and make your changes in that.

This function was missed when merging this pull request, #11