Made scroll bar layzier

  - When the value doesn't change, don't call the callback
  - When the mouse wheel is used and the value doesn't change,
    don't redraw anything
Removed sgui_canvas_draw_box function (no longer needed)
Fixed button rendering
"Ported" group box to skin pixmap and moved rendering code
"Ported" tab caption to skin pixmap

  - Tab caption uses skin pixmap
  - Moved rendering code to tab code
  - Removed get_widget_extents function