Expand ContextMenu item API

ContextMenu should be expanded so desktop apps can build context menus from MenuItem, Menu, and MenuSeparator objects instead of simple text rows. The API should support adding existing items and submenus, inserting and removing items, retrieving items by index, returning the selected item with ShowAtItem(), and reading SelectedIndex or SelectedItem after the menu closes. MenuItem also should support an Icon property used by context menus and in-window menu rendering, making context menus consistent with the menubar API.