expo: Place menu items to the right of all labels

expo: Place menu items to the right of all labels

At present a fixed position is used for menu items, 200 pixels to the
right of the left side of the labels. This means that a menu item with
a very long label may overlap the items.

It seems better to calculate the maximum label width and then place the
items to the right of all of them.

To implement this, add a new struct to containing arrangement
information. Calculate it before doing the actual arrangement. Add a
new style item which sets the amount of space from the right side of
the labels to left side of the items.

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