introspected by checking if (widget_class->set_scroll_adjustments_signal) != 0.

	* gladeui/glade-utils.[ch]: Added glade_util_check_and_warn_scrollable() to check and warn the
	  user if they are trying to add a non-scrollable widget to a scrolled window.

	* gladeui/glade-app.c, gladeui/glade-popup.c, gladeui/glade-placeholder.c, plugins/gtk+/glade-gtk.c:
	Consult glade_util_check_and_warn_scrollable() before executing commands that introduce objects
	to the project.