o removed glade_design_view_get_layout() declaration
    o removed gladeui/glade-design-layout.h include
    o implemented _glade_design_view_freeze/thaw()
    o handle widget adding and removal on project's add-widget and remove-widget signals
    o use widget-visibility-changed to remove toplevel from project when its hidden

  * gladeui/glade-design-layout.[ch]:
    o added design-view construct only property
    o use _glade_design_view_freeze/thaw() to avoid scrolling to selection from
      the layout.
    o removed unused _glade_design_layout_should_scroll()

  * gladeui/glade-design-private.h: added new file to declare private API.