Initial implementation of Cocoa and iOS font loading.

[freakboy3742]

Add font registration to Cocoa and iOS backends.

This supports most use of custom fonts. The only piece missing are variant fonts - fonts where a single font file contains the details for multiple weights/variants. The Endor font is an example of this. Cocoa and iOS will render the font, but not with bold/italic variations, even if the font allows it. Fonts that are non-variant (e.g., the Roboto fonts) aren't affected by this.

My inclination is to say this is complete as a fix for #1837; document the lack of support for variant fonts as a platform detail, and open a ticket to address the missing feature once this lands.

• Fixes Add support for custom fonts on remaining platforms #1837.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[mhsmith]

Cocoa uses the internal font name (how the file describes the font) rather than allowing for an externally registered name.

GTK may have the same issue, though it hasn't been a problem for the tests so far. If necessary, we could find the internal name using the same library as the Android tests:

toga/android/tests_backend/fonts.py

Lines 106 to 107 in 30fabb4

	font = TTFont(family.getFont(0).getFile().getPath())
	assert font["name"].getDebugName(1) == expected

[freakboy3742]

@mhsmith I've taken a swing at getting this working with fonttools - and it mostly works on both iOS and Cocoa. There's still an issue with fonts like Endor that support multiple weights/variants from a single file.

I still think it should be possible to get this working without needing fonttools, but a working solution based on a third party package is a better option than something not working at all.

[freakboy3742]

The macOS build is currently failing because of an issue with binary merging; beeware/briefcase#1483 should address the problem.