gladeui/glade-app.c: Load composite templates from G_USER_DIRECTORY_TEMPLATES

gladeui/glade-project.[ch]
 o Added glade_project_dump_string()
 o Addes safe guards for NULL catalogs (composite template adaptors does not have a catalog)

gladeui/glade-widget-adaptor.[ch]
 o Added template and template-path properties
 o Added glade_widget_adaptor_get_template() and
   glade_widget_adaptor_from_composite_template()

gladeui/glade-composite-template.[ch]: Added support to load composite templates and export a widget as such.

plugins/gtk+/glade-gtk.c, plugins/gtk+/gtk+.xml.in: added "Export as template" action

Fixed bug #664276 "Can't add widgets to a GtkAssistant"
Make assistant update current page when a child is selected in the project.

Fixes bug 684041 "assertion failed: `gtk_accel_group_from_accel_closure (accel_closure) != NULL"

instead load is as an object stub and let the user what to do.
Should be enough to close bug #646259
"SIGSEGV in glade_widget_adaptor_object_construct_object loading ui file"

Fixes glitches when the widget is on a different state.

Tags are broken. Hindi translators must fix it.

symbolic close icon is used for the button.