TrafficControl: Refactoring and User Interface Fixes

After adding a toolBar for hiding/showing the different windows of the program, I need to review and refactor my code.

Before adding a widget that helps the user to create a network file, I need to review/refactor the code and fix some bugs. One of them is that when resizing the dockWidgets for tracks/trains/stations, those widgets reset their sizes immediately.

Another bug is that the buttons in the toolbar aren't changed when the windows are closed using the "x" button on the top right. Fix: Connect the signal visibilityChanged to the same slot as the buttons are connected to. In that slot, update both buttons and 

In this code review, I'll focus on variable names, documentation of functions and whether there is redundant code or not. I'll also omvestogate the need of new test cases.