Invalidate text painting nodes when document text block is updated

The commit fixes text repainting when only formatting of the text is
changed by invalidating the affected blocks which will be repainted
on the next update.

Task-number: QTBUG-36743
Change-Id: I03ba747f9d08a9f49d0a012b8349c89c20dc6b55
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>