ApiActions: Remove child downtimes recursively

Services downtimes scheduled via the `all_services` flag get already
removed automatically when removing their parent downtimes (introduced
with #8913). Now, this commit makes it possible to perform the same actions
for all child downtimes, i.e. not only for those of service objects, but
for all child objects represented in the dependency tree.

lib/icinga/apiactions.cpp

@@ -494,7 +494,7 @@ Dictionary::Ptr ApiActions::ScheduleDowntime(const ConfigObject::Ptr& object,
 				<< "Scheduling downtime for child object " << child->GetName();
 
 			Downtime::Ptr childDowntime = Downtime::AddDowntime(child, author, comment, startTime, endTime,
-				fixed, trigger, duration);
+				fixed, trigger, duration, String(), String(), downtimeName);
 			String childDowntimeName = childDowntime->GetName();
 
 			Log(LogNotice, "ApiActions")