[composer] Change to pos() for item position

Currently QGIS uses QGraphicsItem::transform to position composer items in the scene. This is a little strange since the position can be set directly using QGraphicsItem::pos() / QGraphicsItem::setPos() (possibly these weren't available when the item positioning code was originally written?).  I'm needing to use QGraphicsItem::transform for its proper use - setting item transformations, so this pull request makes sure we use the proper QGraphicsItem pos/setPos methods for positioning composer items. There's no regressions that I've encountered and it also has the benefit of making the code more readable.