macOS: Don't keep WA_MacAlwaysShowToolWindow windows always on top

On macOS if an application is no longer active then it will cause any
tool windows to hide until the application is active again. For
applications that did not want this behavior and thus wanted the tool
window to stay visible, the WA_MacAlwaysShowToolWindow flag is
available.

In order to ensure that this flag is respected, the tool window needs
to have its level changed when the application active status changes.
Once it is no longer active the window needs to be seen as a normal
window, and when it is active then it needs to be set to be a window
that is always on top to get the right behavior.

Due to various bugs in AppKit we need to explicitly order windows
in front during this process, which requires us to then iterate the
windows in back-to-front order. For macOS versions < 10.12 there is
no way to get an ordered list of windows, so we fall back to using
the window creation order.

Task-number: QTBUG-57581
Change-Id: If20b4698616707685f83b1378f87593f8169c8c6
Reviewed-by: Andy Shaw <andy.shaw@qt.io>

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