This is fairly substantial rewrite of the GDK backend for quartz and
renamed to macOS to allow for a greenfield implementation.

Many things have come across from the quartz implementation fairly
intact such as the eventloop integration design and discovery of
event windows from the NSEvent.

However much has been changed to fit in with the new GDK design and
how removal of child GdkWindow have been completely eliminated.
Furthermore, the new GdkPopup allows for regular NSWindow to be used
to provide popovers unlike the previous implementation.

The object design more closely follows the ideal for a GDK backend.

Views have been broken out into subclasses so that we can support
multiple GSK renderer paths such as GL and Cairo (and Metal in the
future). However mixed mode GL and Cairo will not be supported. Currently
only the Cairo renderer has been implemented.

A new frame clock implementation using CVDisplayLink provides more
accurate information about when to ...