Fix #2987: QGroundControlQmlGlobal.h: flightMapZoom should be a double, not an integer

Note that a change in zoom signals a `zoomLevelChanged` event; this in turn triggers `FlightDisplayViewMap` which sets the property changed in this commit to
```
onZoomLevelChanged: QGroundControl.flightMapZoom = zoomLevel
```

This triggers the following message in Application Output: `onZoomLevelChanged: QGroundControl.flightMapZoom = zoomLevel`.

This was the case before and after this fix, I don't know if that's a real issue though

src/QmlControls/QGroundControlQmlGlobal.h

@@ -78,7 +78,7 @@ public:
 
     Q_PROPERTY(QGeoCoordinate lastKnownHomePosition READ lastKnownHomePosition  CONSTANT)
     Q_PROPERTY(QGeoCoordinate flightMapPosition     MEMBER _flightMapPosition   NOTIFY flightMapPositionChanged)
-    Q_PROPERTY(int            flightMapZoom         MEMBER _flightMapZoom       NOTIFY flightMapZoomChanged)
+    Q_PROPERTY(double         flightMapZoom         MEMBER _flightMapZoom       NOTIFY flightMapZoomChanged)
 
     Q_PROPERTY(QString  parameterFileExtension  READ parameterFileExtension CONSTANT)
     Q_PROPERTY(QString  missionFileExtension    READ missionFileExtension   CONSTANT)
@@ -161,7 +161,7 @@ signals:
     void isVersionCheckEnabledChanged   (bool enabled);
     void mavlinkSystemIDChanged         (int id);
     void flightMapPositionChanged       (QGeoCoordinate flightMapPosition);
-    void flightMapZoomChanged           (int flightMapZoom);
+    void flightMapZoomChanged           (double flightMapZoom);
     void experimentalSurveyChanged      (bool experimentalSurvey);
 
 private:
@@ -175,7 +175,7 @@ private:
     bool _virtualTabletJoystick;
 
     QGeoCoordinate  _flightMapPosition;
-    int             _flightMapZoom;
+    double             _flightMapZoom;
 
     SettingsFact    _offlineEditingFirmwareTypeFact;
     FactMetaData    _offlineEditingFirmwareTypeMetaData;