macOS: Disable system provided tabbing by default

macOS 10.12 introduced automatic window tabbing, where the OS provides
tab handling on behalf of the application. The feature defaulted to
being on, and the release notes asked applications that had their own
tab handling to disable the automatic tab handling explicitly.

Qt has its own tab handling, and should therefore disable the automatic
handling. We do this on a per NSWindow basis, using the tabbingMode
property of the window, instead of the global allowsAutomaticWindowTabbing
property on NSWindow, so that individual windows can still opt in to
the automatic tab handling. Doing it via the tabbingMode property
also ensures that we're not affecting the host application's
behavior in a plug-in setting.

Ideally Qt's tab handling should read the userTabbingPreference
property of NSWindow to pick up the system preference choice of
whether to open new windows as standalone windows or tabs, but
this is left for a later change. An alternative would also be to
use the automatic window tabbing instead of our own implementation.

Fixes: QTBUG-61707
Change-Id: Icf2078b63ec16202ff7fd83ea264a48dc61978e1
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

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