Re-select contact with open chat after refreshing list

Each time a new message arrives, the contact list gets reloaded. Currently, this results in the selection of contacts being removed.

https://code.briarproject.org/briar/briar-gtk/blob/c345f861eac8dcd871d3a880ea0c6e4098d1478e/briar-gtk/briar_gtk/containers/main_window.py#L170

    def _refresh_contacts(self):
        # TODO: Keep selected contact with open chat
        self._clear_contact_list()
        self._load_contacts()