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.
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
