ProjectExplorer: Introduce and use a Target::buildSystemUpdated signal

Change-Id: I497d46866146600a1ed9162bd720c574ee4d4769
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/nim/project/nimblebuildsystem.cpp

@@ -179,7 +179,8 @@ void NimbleBuildSystem::setTasks(std::vector<NimbleTask> tasks)
         return;
     m_tasks = std::move(tasks);
     emit tasksChanged();
-    emit target()->targetPropertiesChanged();
+
+    emitBuildSystemUpdated();
 }
 
 void NimbleBuildSystem::setMetadata(NimbleMetadata metadata)