Added <add-child-verify-function> semantics to query the plugin if it's ok to add
	  a said "child" to an adaptors widget "parent" and warn the user about it in context.

	* gladeui/glade-project.c: Check glade_widget_add_verify() when pasting widgets

	* gladeui/glade-command.c: Check glade_widget_add_verify() from glade_command_create()

	* gladeui/glade-popup.c: Allow "Add widget here" action to appear for any project object
	  and rely on glade_command_create() to warn the user via glade_widget_add_verify() if
	  adding the widget fails.

	* plugins/gtk+/glade-gtk.c, plugins/gtk+/gtk+.xml.in: Added <add-child-verify-function> support to
	GtkContainer, GtkMenuShell, GtkMenuItem, GtkToolBar, GtkToolItem, GtkToolPalette, GtkToolItemGroup,
	GtkActionGroup, GtkTextTagTable adaptors.