Stored window backend in base window class
Moved OpenGL code in WinDOS port completely into opengl.c file

  - All OpenGL related code in one file
  - No more ifdef cluttered code in window.c
  - Added checks whether window actually has an OpenGL context
Made OpenGL contexts share resources

  - All OpenGL contexts created through SGUI now share resources
  - Easier for users: All OpenGL objects created for one OpenGL window
    or OpenGL widget can be used with others.
  - Can be used to better integrate into SGUI: OpenGL contexts share
    resources, just like the native API (e.g. X11 pixmap is valid on
    every window created through SGUI)