Sample windowing and OpenGL code

Code in a preliminary stage to be reorganized into the Window class.
Currently, OpenGL is working, which can be shown by changing the
parameters in glClearColor() on line 113 in startup.cpp and running the
program - the colour of the window will change accordingly.
Construction of Window and CobaltGame classes

Window class mostly completed; now we have a simple interface to make a
window with an OpenGL context. The CobaltGame class was also added,
which is basically where all of the programmer's code goes when they
want to make a game.