The WidgetWatchers struct is designed to be embedded into custom widgets
in order to satisfy the WidgetWatcher interface. The implementation uses
an internal map for storing EventHandlers, but does not lock access to
the access of the map within each method. A race condition can occur if
Watch, Unwatch, or any event is Posted from separate goroutines.
Lock access to the internal map. When posting an event, create a deep
copy of the map for iterating. This prevents potential deadlocks from a
widget calling Unwatch while handling an event.