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