Update macOS iconset

[patmauro]

Makes a few improvements to the macOS iconset for readability:

• Reverts to original Cyan book graphic(s), rather than our upscaled version, at sizes 256 and below, as these original graphics are already optimized to read best at their respective sizes.
• The smallest icon (16x16) uses a special variant with a much bigger book against a simple gradient rather than a smushed re-use of the same background. This will read much better as an icon at this size.
• The 3 largest icons in the icon set are unchanged, and thus are not included in this PR.

Note: I noticed that a 512px icon a little more accurate to Cyan's original asset may be possible to generate with a little work. However, this doesn't seem strictly necessary at this time; the assets we have in place are more than sufficient. But I may take this as a follow-up item in the future.

[colincornaby]

These changes look good. I especially like the extra detail on the smaller icon.