Mac: Fix mixing unified and non-unified toolbars

Test if the target toolbar is actually a unified
toolbar before calling the special MapFromParent
function.

This change is not needed in Qt 5. 

Task-number: QTBUG-36100
Change-Id: I37fce89c79f5fce057e0b31d4c4f2e72269cdb06
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>