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