Qt::Tool windows float in front of all applications in Mac Cocoa

A QWidget shown with Qt::Tool floats in front of the windows of all applications on the Cocoa version of Qt, This makes it impossible to have floating palettes in Mac Cocoa applications. The Carbon version has correct behaviour, but Cocoa is required for 64-bit applications, so this is not a valid workround.

This is a general problem with other parts of Qt. The Qt::WindowStaysOnTopHint for example has different behaviour on different platforms - sometimes in front of the app's windows, sometime in front of all apps' windows. Ideally, there should be an equivalent of the Mac's window layering API where we can specify either type of behaviour. Failing that, the floats-in-front-of-own-windows behaviour is far preferable.