Ideally the child widget should not have to know its parent's API, so let's remove a few more unnecessary connections.

• show_error and show_message are provided by MessageBoxMixin, which ContactList inherits from
• the QApplication instance can be fetched directly through QWidget
• main_window.get_wallet_folder() is a one-liner that only needs self.config, which ContactList already has a reference to
• the parent parameter for new contact dialog can be get via the self.top_level_window(), provided for some reason by MessageBoxMixin

In the contacts tab, try all the context menu actions:

• add contact (try with a valid address and an invalid addresse to trigger an error dialog)
• export and import contacts (try also importing an invalid contacts file)
• with one or multiple contacts selected, try "Copy Label" and check it is in the clipboard