macOS: Update font to match system font (Helvetica Neue)

[ChrisNZL]

Also seems to be a bug where setting the global font name isn't applying to results properly on macOS.

[ChrisNZL]

Fixed in 1.6.0.

While I implemented "Helvetica Neue", which is macOS' font from everything I've read, using font:menu seemed to give a different font that looked much better on the eyes. I assume it's the same font, or a variation at least, but I could not figure out how to display it using font-family, so for the justification of making things look nice on macOS I've:

• Used font:menu in a lot of places.
• Removed the ability for a custom font to be specified on macOS.

Things were looking very cramped before too, so I've increased the white space padding on the input fields and results, and increased the font size slightly.