expo: Set up the width and height of objects

expo: Set up the width and height of objects

Provide a way to set the full dimensions of objects, i.e. including the
width and height.

For menus, calculate the bounding box of all objects in the menu. Set all
labels to be the same size, so that highlighting works correct, once
implemented.

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