Updates regarding display class.

 - Basic display functionality, except for drawing, implemented using Freeglut and GLEW.
 - Display class management handled by main application, not Application class.
 - Application instance registered on Display class for objects rendering.
 - Display accesses from Application class were performed by Instance() calls.
 - Some TODO statements were discarded (leaving only related comments).
 - Display ::clear() and ::refresh() function calls moved from Application class to Display class.
 - Display related initialization removed from Application::initialize() method, instead setupObjects() called.
 - Application::cleanup() method removed along with display related calls.
 - Quit request processing removed from Application and implemented inside Display class.
 - Internally used methods changed into private methods.
 - Some new Display methods defined.
 - Display::open() function now performs all display initialization, including shaders.
 - Makefile updated to build with glut and glew.