Allow customizing the apparence of a window independently of its behaviour

It would be nice if you could add a couple of flags to Qt::WindowFlags, or widgetAttribute, allowing to customize the apparence of a window independently of its behaviour.

In particular:
1/ Move MacAlwaysShowToolWindow from WidgetAttributes to WindowFlags, by adding a flag Qt::HideOnAppDeactivation

2/ Add a Window::Flag Qt::AlwaysOnTopOfAppWindows (which differs from Qt::WindowStaysOnTopHint, since the latest acts throughout all the windows of all appplications).

3/ Add a Window::Flag Qt::ToolDecorated, to refer to small tool window decoration (when available on the underlying system).

4/ Change Qt::Tool to define it as
Qt::Tool = Qt::ToolDecorated