TB 43405: Split TorProvider writeSettings into separate methods for the
proxy, firewall and bridges settings.

We also call TorSettings.setTorProvider instead of
TorProvider.writeSettings so that TorSettings can handle the application
errors.

TB 43405: Do not allow string values for proxy and firewall ports. And
do not allow a proxy username without a password or vis versa.

TB 43405: TorSettings handles failures to apply Tor settings.

We update TorSettings.#applySettings to catch TorProvider write errors
and signal this error with "ApplyError".

We also keep track of which group of settings have failed so that we can
restore them on the user's request.

fixup! TB 31286: Implementation of bridge, proxy, and firewall settings in about:preferences#connection

TB 43405: Add some validation to the Advanced connection settings
dialog.

TB 43405: Add strings for tor settings error notification.