SettingsAbstractWidget can have specialized event ids

SettingsAbstractWidget can have specialized event ids for their own
setting changes to be broadcasted to all NotificationHub listeners.

Previously, any setting change automatically triggered a broadcast of
NotificationHub::EventConfigurationChanged, but now each widget can
state which specific event shall be published if it changes.
The current implementation is quite basic and still signals
NotificationHub::EventConfigurationChanged on any change, i.e. none of
the descendants of SettingsAbstractWidget states a specialized event.