QMessageBox - move layout handling

This patch moves some layout handling to a new function.
That will be helpful later since we can re-use the new function.

Change-Id: I4cc846f9958d9530ec2b07292093b94bd27ee055
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
QMessageBox - merge some mac and non-mac code

I have not located any significant behavior changes in this,
and it is annoying having too much similar code in different
branches.

The main change is a removal of Qt::AlignLeft, but
the icon Label should be in its own column.

Change-Id: Iaf07fc503075e61e2af1a93d5fc9e6e34a24451d
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>