Mat conversions for macOS/AppKit

* Extract CoreGraphics conversion logics from ios_conversions.mm to apple_conversions.h, apple_conversions. Add macosx_conversions.mm

* Add macosx.h

* Add Mat+Conversions.h and Mat+Conversions.mm

* Delete duplicated declaration from apple_conversion.mm

* Use short license header

* Add compile guard

* Delete unused imports

* Move precomp.hpp import from header to implementation

* Add macosx.h to skip headers

* Fix compile guard condition

* Use short license header

* Remove commented out unused code