TrafficControl: Minor Fixes for Network Designer and User Interface

As mentioned earlier, the default path to the network files were hardcoded. This made TrafficControl dependent on having that particular folder on the particular computer, which is bad design. Instead, I used QStandardPaths to make the program look for the files in the user's home directory.

Further, I've modified the code for generating the TNM file. It is possible to select either a KML file, or a Traffic Network Map/File. In the latter case, the file is opened and parsed directly to the text field.

TrafficControl is setting up the UI and has many signal/slot definitions. I moved those to a separate function.

Finally, I deleted some objects in the trafficControl destructor in order to prevent memory leaks.

In the future, I'll need to add tet cases and error handling.