macos: prototype new GDK backend for macOS

macos: prototype new GDK backend for macOS

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 draw drawing the next frame. Some
testing will need to be done here to understand the power implications
of this.

This implementation has also gained edge snapping for CSD windows. Some
work was also done to ensure that CSD windows have opaque regions
registered with the display server.

     ** This is still very much a work-in-progress **

Some outstanding work that needs to be done:

 - Finish a GL context for macOS and alternate NSView for GL rendering
   (possibly using speciailized CALayer for OpenGL).
 - Input rework to ensure that we don't loose remapping of keys that was
   dropped from GDK during GTK 4 development.
 - Make sure input methods continue to work.
 - Drag-n-Drop is still very much a work in progress
 - High resolution input scrolling needs various work in GDK to land
   first before we can plumb that to NSEvent.
 - gtk/ has a number of things based on GDK_WINDOWING_QUARTZ that need
   to be updated to use the macOS backend.

But this is good enough to start playing with and breaking things which
is what I'd like to see.