Notify plugins on certain events (e.g. body added).

This also adds a periodic tick function to plugins, giving them periodically control over the simulation thread, even when the simulation is paused.

examples/SharedMemory/plugins/b3PluginContext.h

@@ -16,6 +16,8 @@ struct b3PluginContext
 	int m_numKeyEvents;
 	const struct b3MouseEvent* m_mouseEvents;
 	int m_numMouseEvents;
+	const struct b3Notification* m_notifications;
+	int m_numNotifications;
 };