This commit fixes the singlePar optimization which was disabled due to the ParagraphM...

This commit fixes the singlePar optimization which was disabled due to the ParagraphMetrics cleanup.

* rowpainter.C::paintPar(): don't calculate row signature here. Use ParagraphMetrics::rowChangeStatus() instead.

* RowList_fwd.h: RowSignature is now a vector.

* ParagraphMetrics
  - rowChangeStatus(), updateRowChangeStatus(), calculateRowSignature(): new methods.
  - row_signature_: new member
  - rowSignature_: renamed to row_signature_

* TextMetrics::redoParagraph(): call ParagraphMetrics::updateRowChangeStatus()

* BufferView::updateMetrics(): simplify the singlePar case.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@16543 a592a061-630c-0410-9148-cb99ea01b6c8