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
