expo: Support handling any key in cedit

At present cedit only supports menu keys. For textline objects we need
to insert normal ASCII characters.

We also need to handle backspace, which is ASCII 9.

In fact, expo does not make use of all the menu keys, so partition
them accordingly and update the logic to support normal ASCII
characters, too.

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