Renamed PositionDataMessage class to simply PositionData.

I think it would be a good idea for our message class names to match the name of the corresponding protobuf message. This will allow us to use the Qt "metaobject" to extract the name of the class as a string when we need to present it in the GUI. E.g. we could do something like

```cpp
QString Message::name() {
    return metaObject->className();
}
```

and magically, all our Message subclasses would have this name() function available to use when presenting messages in the GUI.

Hence, this pull requests renames the PositionDataMessage class (our only Message subclass so far) to simply PositionData.