Improve chapter variable again.

1) On top-down used for referenced layouts go first from the page's top
down to it's bottom and if we didn't found something continue search
backwards at the pages top.

2) Handle the case that the QTextFrame::iterator doesn't point to a
block but a frame.

3) Don't execute KoVariable::resize once setValue is called cause it
ends in the variable beeing to small if it's in the last line of
a header/footer for whatever reason.

Related to bug 276262.