Decrease code duplication by reusing QTextEngine::findItem()

Use the findItem()'s optimized binary search impl instead of a generic one.
Also drop the "already shaped item" case since setBoundary() is only
called from itemize(), where the items are not shaped yet.

Change-Id: Ifbecdc41b2e3cb7791a7896fbb0cbea439ca0706
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>