gcc has optimizations for include guards that only work
if they are outermost in the the header.
https://bugzilla.gnome.org/show_bug.cgi?id=689810

  - 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

This reverts commit d32d7c8f9c4d2bcd7c5c206c09273ce67ed20df4.

Added properties "wrap-width", "row-span-column" and "column-span-column"
to allow grid style menus from treemodels. Handling row data changes
appropriately.

Combo boxes expose an "add-tearoffs" feature to add a tearoff item to the root of the combo menu, added this feature to GtkTreeMenu to achieve this (and updated the testcase).

GtkComboBox needs treemenus to allow selection of all leafs including
rows which may have children, this allows the combobox or combobox user
to decide which row that has children can also be selectable as a header
leaf of the submenu. Test case testtreemenu updated to reflect this.

GtkTreeMenu needs to only populate it's submenus when set_root()
is called, we were populating it when the model is set which cause
the tree to be infinitely populated as the root is NULL by default.

Also call gtk_menu_set_reserve_toggle_thingy (FALSE) to not reserve
space for the toggle size.

Added GtkTreeMenu class to automatically render
a GtkTreeModel into a GtkMenu hierarchy (will be
used by GtkComboBox for its dropdown menus). Included
an accompanying testcase tests/testtreemenu