+semver:major [SIL.WritingSystems] Add UI font role

[imnasnainaec]

Resolves #1212 (implementing #1212 (comment))

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This change is 

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Palaso Tests

     4 files       4 suites   16m 20s ⏱️
 4 902 tests  4 674 ✅ 228 💤 0 ❌
15 957 runs  15 273 ✅ 684 💤 0 ❌

Results for commit ff71933.

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[imnasnainaec]

@ermshiperete For the roundtrip tests to work with XML files that have a font without any type specified, I thought it would make sense to change the code to default to FontRoles.None rather than FontRoles.Default. However, I'm not sure if there are usages of the library that would reasonably expect or rely on (e.g.) all write-outs some type specified on every font.

[ermshiperete]

I don't know the answer to that question and I don't know where that class is used. You might want to ask on Slack.

[imnasnainaec]

I don't know the answer to that question and I don't know where that class is used. You might want to ask on Slack.

No responses in #commonlibraries; should I ask again in LT Software's #public or LT's #general?

[ermshiperete]

No responses in #commonlibraries; should I ask again in LT Software's #public or LT's #general?

Everyone who cares should watch #commonlibraries; otherwise it's documented as breaking change in the changelog, so I think we're good.

[imnasnainaec]

Reviewed 5 of 6 files at r2, 1 of 1 files at r4, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @imnasnainaec)