* Allow to select units in components that have more than one right in the
component chooser dialog.
* Keep chosen unit in history.
* Show preview of current component unit as thumbnail image next to the
description box.
* Fixes lp:1280567
* make some more fields 'const' that can.
* Instead of previous/next _visible_ element, Go through
previous and next element. Otherwise the cursor stops moving
if the item is only partially visible.