Add a signal to notify when an extension's settings are saved.
A pattern with extensions was to have an extension's SettingsForm

subclass call into the Extension when the form saved, in order to

operate on the new values. However, that only really covers when

SettingsForm saved, and wasn't really the best way to do things.
This change introduces a new signal, settings_saved, which is emitted

whenever an extension's settings have been saved. Extensions can listen

to this, with 'self' as the sender, in order to operate on any new

settings.

Used this in an extension I'm building. Verified that whenever I clicked

Save, my callback handler was called.
Unit tests pass.