Groundwork for GUI system

- Extended GraphicsManager to handle vertex buffer rendering
- Created base IGUIWindow class. All gui elements will inherit from this
class.
- Fixed character support for InputManager
More GUI functionality

- Refined rendering process for GUI
- Added support for new GUI types (GUIBox and GUIButton)
- GUIButtons now fire OnClick() when clicked on, and change colour for
visual feedback
Added files missed in previous commit
Merged gui into master