Added OpenGL view widget

(renamed "sub view" after realizing that it can be used more general)

  - Widget that creats and manages a subwindow within a window
  - Had to implement widget state change callbacks and tab
    select/deselect callback to force widget behaviour on window
  - Redraw on demand (render callback called when subwindow triggers
    expose callback)
  - User callbacks can listen to normal subwindow events
  - Still absolute window coordinates (does not use to parent widget
    relative position, but parent window relative position)

TODO:

  - Implement continuous redrawing (seperate thread with rendering loop)
  - Ability to force redrawing
  - Use parent widget relative coordinates