Definition of Constants for UI modification

Added constants and adjusted code for rendering and hit checking to
allow for easier and safer development and use of the Main Menu UI
buttons.
- The rendering and hit checking of buttons on the Main Menu now scale
together
- Adding additional (unlabelled) buttons to the Main Menu doesn't
require changes to the rendering code nor code to check explicitly for
the new button being pressed.
gitignore

Adding gitignore file to ignore generated files