instead of relying on being child of a GtkToolbar. (#5034079)

* gtk/gtk.symbols, docs/reference/gtk/gtk-docs.sgml,
docs/reference/gtk/gtk-sections.txt: Add GtkToolShell symbols.
* docs/reference/gtk/tmpl/gtktoolitem.sgml: Move section docs inline.
* gtk/gtktoolbar.c: Implement GtkToolShellIface.
* gtk/gtktoolbar.h: Remove _gtk_toolbar_rebuild_menu.
* gtk/gtktoolitem.c: Use GtkToolShell, instead of GtkToolbar.
Take section docs from template file and update them for GtkToolShell.
* gtk/Makefile.am: Add gtk/gtktoolshell.c and gtk/gtktoolshell.h.
* gtk/gtktoolshell.c, gtk/gtktoolshell.h: New GtkToolShellIface.

svn path=/trunk/; revision=19177