ProjectExplorer: Introduce and use a Target::buildSystemUpdated signal

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

src/plugins/cmakeprojectmanager/cmakebuildsystem.cpp

@@ -362,6 +362,8 @@ void CMakeBuildSystem::combineScanAndParse()
     }
 
     m_currentGuard = {};
+
+    emitBuildSystemUpdated();
 }
 
 void CMakeBuildSystem::checkAndReportError(QString &errorMessage)