Assistant: Introduce a 'compact' display for the simple search

Setting the mode disables the Previous and Next buttons for the simple
search history. This is meant to be used e.g. in Qt Creator where the
horizontal space in the side bar is precious.

Change-Id: If13ffbf2656c471ceb10c5bb15e39166fedbcb85
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>