[Xwt.Mac] Fix ToCGColor utility method to use the same colorspace than ToNSColor.

CGColor and our NSColor defaults to two different colorspaces which produces different colors when displayed on screen.
[Xwt.Mac] Make Xwt.Mac widgets uses CALayer and provide a default implementation of BackgroundColor using it.
[Xwt] Backport performance improvements to Table
[Xwt.Mac] Don't scale images in NSButtons and set the mode to image-only when there is no label