Display version number in toolbar on the settings page

Fixes #341

app/backend/systemproperties.h

@@ -21,6 +21,7 @@ public:
     Q_PROPERTY(QString unmappedGamepads MEMBER unmappedGamepads NOTIFY unmappedGamepadsChanged)
     Q_PROPERTY(int maximumStreamingFrameRate MEMBER maximumStreamingFrameRate CONSTANT)
     Q_PROPERTY(QSize maximumResolution MEMBER maximumResolution CONSTANT)
+    Q_PROPERTY(QString versionString MEMBER versionString CONSTANT)
 
     Q_INVOKABLE QRect getDesktopResolution(int displayIndex);
     Q_INVOKABLE QRect getNativeResolution(int displayIndex);
@@ -44,5 +45,6 @@ private:
     QSize maximumResolution;
     QList<QRect> monitorDesktopResolutions;
     QList<QRect> monitorNativeResolutions;
+    QString versionString;
 };