* gladeui/glade-design-layout.[ch]:
    o added widget maximization on label double click
    o fixed size allocation to be able to maximize widget without making the viewport
      scrollbars appear
    o removed glade_design_layout_selection_set()

  * gladeui/glade-design-view.c:
    o improved viewport scrolling to selected toplevel, now we only scroll if
      the selection was not triggered by the layout.
    o replaced on_project_add_widget() on_project_remove_widget() with new project
      signal widget-visibility-changed

  * gladeui/glade-project.[ch]:
    o added GladeProject::widget-visibility-changed signal
    o added glade_project_widget_visibility_changed() function

  * gladeui/glade-widget.c:
    o added visible property.
    o use glade_project_widget_visibility_changed() to emit signal on widget visibility change

  * gladeui/glade.h: removed glade-design-layout.h since now its private

  * gladeui/Makefile.am: do not install private header glade-design-layout.h

  * plugins/gtk+/Makefile.am: do not install headers