Song: reset global models in clearProject()

We have to reset global models such as tempo, master volume etc. before
clearing the global automation patterns attached to them. Fixes bug with
persistent automation on these controls when clearing project.

Closes #3046039.

src/core/song.cpp

@@ -757,6 +757,11 @@ void song::clearProject()
 		engine::automationEditor()->setCurrentPattern( NULL );
 	}
 
+	m_tempoModel.reset();
+	m_masterVolumeModel.reset();
+	m_masterPitchModel.reset();
+	m_timeSigModel.reset();
+
 	AutomationPattern::globalAutomationPattern( &m_tempoModel )->clear();
 	AutomationPattern::globalAutomationPattern( &m_masterVolumeModel )->
 									clear();