Emit updateBlock signal in QTextDocumentLayout

updateBlock is part of the interface of QAbstractTextDocumentLayout
however QTextDocumentLayout implementation is not invoking it.

This commit adds similar behavior to QTextDocumentLayout as the
QPlainTextDocumentLayout (QtWidgets) has by implementing the missing
functionality.

[ChangeLog][QtGui][QTextDocumentLayout]
Emit updateBlock signal in QTextDocumentLayout.

Task-number: QTBUG-36743
Change-Id: I7f812d4d5d7c2148393ee672fb0db6c0b2e73783
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>