Added some more UI features and fixed misc. bugs

So I showed this to some of my friends and got some very helpful feedback especially on UX.

Feature improvements:

• In the simulate window, I added a back button to go back and cancel the simulation so you no longer have to wait for the simulation to finish.
• When you are building the start of the road, it shows a preview of where the road will be to help with snapping

Bugfixes:

• My buttons were previously accepting any clicks, even when the focus was not on the application, I fixed that by checking the focus of the application before accepting mouse clicks.
• Road snapping stayed the same distance from intersections even though there might be cases when the user wants to create a road near another road but not directly adjacent. I fixed this by multiplying the snap distance by the zoom factor to get a new corrected version.

Overall, I don’t plan to make many more changes and will probably just add another 3-5 levels, work on the title screen, and get the game ready for shipping.