macOS: End support for modal native color dialogs

The native macOS color dialog is a modeless panel; We've made it work with the modal exec()/getColor() API in Qt but this is somewhat of a hack which keeps breaking. See QTBUG-66101, ...

This has proven to be non-maintainable so we want to:

1) Deprecate modal usage and print a warning (and close bugs as out-of-scope).

2) Make modal use fall back to use the Qt dialog.

Applications will then have the choice of either writing code to support the non-modal work flow, or keep existing code and use the Qt dialog.