Make SystemMonitor API more declarative

The following functions should be converted into regular properties:

bool setMemoryWarningThresholds(qreal lowWarning, qreal criticalWarning);
qreal memoryLowWarningThreshold() const;
qreal memoryCriticalWarningThreshold() const;

bool addIoLoadReporting(const QString &deviceName);
void removeIoLoadReporting(const QString &deviceName);
QStringList ioLoadReportingDevices() const;