o implemented get_preferred_width_for_height() and get_preferred_width_for_height()
      to fix allocation problem
    o check if toplevel has a child selected before selecting it itself on button press
    o dont draw selection if widget has no allocation

  * gladeui/glade-design-view.c:
    o delegate scrolling to size-allocate if toplevel layout has no allocation
      Fixes scrolling while creating a new widget from the palette.
    o also add widgets on visibility change