Using NSScreen as the basis for tracking screens is not recommended, as
the list of screens can be added, removed, or dynamically reconfigured at
any time, and the NSScreen instance, or index in the NSScreen.screens array
may not be stable.
Quartz Display Services on the other hand tracks displays via a unique
display ID, which typically remains constant until the machine is restarted.
The lower level API also gives us earlier callbacks about screen changes
than the corresponding NSApplicationDidChangeScreenParametersNotification
does. By reacting to screen changes _before_ AppKit does, we can remove
workarounds for receiving window move and screen change notifications
before the screen was actually visibly reconfigured.
The new approach also handles changes to the primary screen, which
can happen if the user moves the menu bar in the macOS display
arrangement pane.
The device pixel ratio of the screen has been made into a cached
property, like all the other properties of QCocoaScreen. This is
more consistent, and allows us to qDebug the screen even when it
has been removed and we no longer have access to resolve the
properties from the associated Quarts display.
Change-Id: I2d86c7629ed3bf5fb8c77f174712633752ae4079
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>
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