fix transient dialog placement
fix transient dialog placement

dialogs can be bigger than the mainwindow and the unsigned dimensions
then overflow in the subtraction (the window would still be moved into
screen bounds but appear on ugly 0,0)

Arkq/fluxbox,luebking/fluxbox,Arkq/fluxbox,Arkq/fluxbox,luebking/fluxbox,luebking/fluxbox,Arkq/fluxbox,luebking/fluxbox,Arkq/fluxbox,luebking/fluxbox