AutomatableModel: added centerValue property

The optional centerValue property can be used by views to display the
center somehow. It defaults to minimum value.

src/core/AutomatableModel.cpp

@@ -45,6 +45,7 @@ AutomatableModel::AutomatableModel( DataType type,
 	m_maxValue( max ),
 	m_step( step ),
 	m_range( max - min ),
+	m_centerValue( m_minValue ),
 	m_journalEntryReady( false ),
 	m_setValueDepth( 0 ),
 	m_hasLinkedModels( false ),