[electrum] remove more uses of main_window in ContactList
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Description

[electrum] remove more uses of main_window in ContactList

Summary:
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

Test Plan:
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

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien