gtkmodelmenu: simplify logic, expose bind API

Make the main (and only) entry-point to gtkmodelmenu.c the now-public
gtk_menu_shell_bind_model().

Move the convenience constructors (gtk_menu_new_from_model() and
gtk_menu_bar_new_from_model()) to their proper files.

Remove the private header file.

Simplify the code a bit by making the initial populate part of the
bind() call.

https://bugzilla.gnome.org/show_bug.cgi?id=682831

docs/reference/gtk/gtk3-sections.txt

@@ -2246,6 +2246,7 @@ gtk_menu_shell_set_take_focus
 gtk_menu_shell_get_take_focus
 gtk_menu_shell_get_selected_item
 gtk_menu_shell_get_parent_shell
+gtk_menu_shell_bind_model
 GtkMenuDirectionType
 <SUBSECTION Standard>
 GTK_MENU_SHELL