FontAwesome 5.6.1, fix XML docs, unit tests & concurrency bug fix.

[johanbenschop]

Merry Christmas! The primary change in this pull request is the update of FontAwesome to version 5.6.1.

Also:

• Improved the generator which now directly pulls from GitHub and updates the files in one fell swoop (given FontAweomse doesn't change their folder structure).
• Now including the DocumentationFile inside the NuGet package so consumers get the documentation of the enums.
• Improved the unit tests to use XUnits MemerData functionality to generate tests per value in an enum.
• Two types of tests, one for embedded and one for custom fonts. Total 4308 tests ~50 seconds to run them.
• Tests now count the unique colors per image to verify the icon could be found.
• The tests give a code coverage of 55.73%.
• While making these tests I got random exceptions with something about an error in GDI+ when running tests in parallel with multiple font styles. Solved this by adding a MemoryCache to the LoadFontFamily methods in the FontThumbnailService.
• While debugging I also improved the code, like adding in more using statements for disposing of objects.

I look forward to feedback (if any).

[degborta]

Merry christmas @johanbenschop and thanks for the contribution. It looks really nice! Only stumbled on one version conflict that I added a suggestion for. Take a look and get back with your comments.