expo: Support drawing of popup menus

expo: Support drawing of popup menus

At present only a single menu is supported. All items are shown and a
pointer object points to the current item.

Add support for multiple menus, one of which is highlighted, indicated
by the highlight_id property in the scene.

The highlighted menu item has a SCENEOF_POINT flag, indicating that it
is currently pointed to.

The popup menu is normally closed, in which case it shows only the
current menu item. When it is opened, it shows all items, allowing the
user to select one.

Rather than requiring the menu item to have a description, require it to
have a label. Use the label (only) for the popup menu.

With this, most of the drawing and layout logic is complete.

Signed-off-by: Simon Glass <sjg@chromium.org>