Turn setUnifiedTitleAndToolBarOnMac into a slot.

Change-Id: Ieb28960bf3fb9e70a6bcc6d47ff723f0ed4e1b63
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>
Add a menu item to mainwindow example to toggle unified toolbars.

Change-Id: Id7d7433849565b1acfc690cf5e23e8105c6af993
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>