Minor optimization for QTextLineItemIterator

Don't store unused values (pos_x and levels) and re-use
already calculated ones (itemStart, itemEnd, and itemLength).
Also const-ify some members to make the code a bit more clear.

Change-Id: Ied80ebf9e4e7e8a1d057e413a9bd24f84b8aaf92
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>