C++ Qt Game Tutorial 9 - Starting a Tower Defense Game

We start creating a Tower Defense Game. We review several important concepts and learn several new concepts. New concepts include QVector, QPoint, QLine, QPolygon, mapping from one coordinate system to any other, QLine::dx(), and QLine::dy(). Code for this tutorial: https://github.com/MeLikeyCode/QtGameTutorial/tree/master/tutorial9