extract and optimize BlockInlineIterator

BlockInlineIterator recreates and reiterates the children axis in every
step. As optimization a stack is introduced which carries the currently
used iterator. With a call to next, this iterator is reused and the
iteration continues at the former position.

FormattingPortionIterator is a better name.

Change-Id: I9f5669934e25da53e049e6c0cd6cba913b3f03a8
4 files changed