Added things like a Window Size Change Listener
Full list of changes below VV

- moved the adding of JPanels to the window class
- added method to add and remove mainGUI
- added update method to mainGUI to be called by window class
- added update method to window class to update any GUI
- added a Resize Listener for window class (soon to be for any JFrame
  class)
- made components update position based on and when screen size changes
- added method to add / remove any GUI to the window
- added method to switch from a specified GUI to another
- added method to remove any GUI currently on window