Make GtkTreeMenu a private class:

Make GtkTreeMenu a private class:

  - Removed GtkTreeMenu from gtk-docs.sgml
  - Removed GtkTreeMenu from gtk3-sections.txt
  - Removed GtkTreeMenu from gtk.symbols
  - Make GtkTreeMenu apis prefixed with '_' (including _get_type()).
  - Updated GtkComboBox sources to use the private apis
  - Updated GtkCellView to not mention #GtkTreeMenu in gtk-doc statements
  - Updated tests/testtreemenu to not use a GtkTreeMenu but still
    show a very fancy GtkComboBox
  - Moved gtktreemenu.h to private headers section in the makefile.
  - Removed include of gtktreemenu.h from gtk.h