Replace SFML\ with SFML/ in includes

\ as a path delimiter only works on windows, / works on both windows and linux
include <cmath> where required
Game.cpp: Use sf::Mouse::Left instead of sf::Mouse::Button::Left

enum Button in sf::Mouse is not a scoped enum.
Fix non-matching case in some referenced filenames

Linux file lookup is case sensitive
Add CMakeLists.txt

This allows people who have CMake to easily build the project.