Added tab group offset

  - Tab gap left/right skin elements in tab group are now used
    for offsetting the tab captions, e.g. if there are round
    corners
  - Still need to handle case for the right hand side
Removed OpenGL canvas implementation

Considered "beyond the scope of this project". A memory canvas will be
implemented in the future to be used by the windows backend and to draw
to OpenGL textures.