Implemented preview feature for Glade.

	* gladeui/Makefile.am, gladeui/glade-previewer.c: Added program to preview
	  glade files.

	* gladeui/glade-project.[ch]: Implemented glade_project_preview() to spawn a
	  child preview process, reap it when it dies and close all previews when
	  project closes.

	* src/glade-window.c: Added toolbar button to spawn the preview.

	* gladeui/glade-xml-utils.[ch]: Added glade_xml_dump_from_context().

	* plugins/gtk+/glade-gtk.c, plugins/gtk+/gtk+.xml.in: Added actions to preview a widget.

	* gladeui/glade-app.[ch]: Added glade_app_get_bin_dir() for resolving of previewer path.