|SOURCE|
[Linux::GLWindow]
* Initialise accepts a function pointer to a callback function which handles
  application shutdown when the window is closed
* The window reports being closed via the window manager's delete/close button
* Update checks for if the 'q' key is pressed (temporary) and calls the close
  function registered in Initialise
[Linux::Main]
* Added a function which modifies a global variable for tracking if the program
  should shut down, this function is registered in the Initialise member
  function of the GLWindow instance