POS: Option to show the item's image and details

Add an option to show the item's image and details on POS, making it easier to
identify the selected product. The details are hidden by default in an expander
to avoid interface pollution.

Also, simplify the SellableImageViewer code a little.

Change-Id: Ica8262791d8c5c88870ed1652a31b223e27f46c7
Fixes: http://bugs.async.com.br/show_bug.cgi?id=5571