Expose the number of matched sensors on aggregate sensors

So it can be used when calculating values for aggregates.

systemstats/AggregateSensor.cpp

@@ -174,6 +174,11 @@ void AggregateSensor::removeSensor(const QString &sensorPath)
     }
 }
 
+int AggregateSensor::matchCount() const
+{
+    return m_sensors.size();
+}
+
 void AggregateSensor::updateSensors()
 {
     if (!m_matchObjects.isValid()) {

systemstats/AggregateSensor.h

@@ -54,6 +54,8 @@ public:
     void addSensor(SensorProperty *sensor);
     void removeSensor(const QString &sensorPath);
 
+    int matchCount() const;
+
 private:
     void updateSensors();
     void sensorDataChanged(SensorProperty *sensor);