Bug 42036: Refactor TorSettings.

We make sure we send out a notification every time a TorSettings setting
property changes.

We also place all string parsing in one place (the "transform" method).

We also use empty strings "" instead of null. The previous null values
would have been saved to preferences as an empty string and read back as
an empty string, so this keeps it consistent.

Enforce the "enum" types to be one of the existing values.

Use structuredClone for copying the _settings object.

Stop using console logging.

Stop exposing the proxy port setting as an array of one number. This
only worked before because a JavaScript array with one member converts
to the same string as the member itself.

Stop resetting the proxy settings when proxy.enabled is set to true
since all places that call this should set the other settings afterwards
anyway.

Stop setting firewall.allowed_ports to 0 rather than [].

Fix setting username and password for HTTP proxy.uri. Before it would
only do so if _proxyUsername was truthy, but this property was never
set.

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

Bug 42036: Adjust for changes in TorSettings.

New API for SettingChanged notification.

Unset the username and password for SOCKS4 proxies explicitly in the
dialog settings.

Bug 42036: Adjust for changes in API for TorSettings SettingChanged
notification.

Bug 42036: Remove reference to "self" in TorSettings, and re-indent.