ProjectExplorer: Introduce and use a Target::buildSystemUpdated signal

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

src/plugins/genericprojectmanager/genericproject.cpp

@@ -498,6 +498,8 @@ void GenericBuildSystem::refresh(RefreshOptions options)
     refreshCppCodeModel();
     updateDeploymentData();
     guard.markAsSuccess();
+
+    emitBuildSystemUpdated();
 }
 
 /**