Decouple Label and TextColorStyle

[amgleitman]

Platforms Impacted

• iOS
• macOS

Description of changes

NOTE: This is currently targeted for new-2ltv, but it will be merged directly into the branch that's part of microsoft#1731 after #1 makes it in.

The discussion of microsoft#1731 uncovered a bug where label tokens don't always play nicely when placed inside parent components. The ultimate problem here is that right now, a Label must be defined in terms of a TextColorStyle. This not only limits what we and consumers of FUA can do with Labels, but it's also an unnecessary abstraction since we usually specify labels in terms of individual typography styles and colors.

Our solution is to make it such that Label is more closely connected to UIColor. This gives us more flexibility in terms of what colors a label can be. It also removes the need for TextColorStyle.secondaryOnColor, which we needed to add as a special case for TwoLineTitleView.

Note that we specify the color as a (FluentTheme) -> UIColor instead of just a UIColor because brand colors (as opposed to neutral colors like "Grey 42" and shared colors like "Blue Primary") require a theme in order to be translatable to a UIColor. See the usage in LabelDemoController.swift for an example.

Verification

(how the change was tested, including both manual and automated tests)

Visual Verification

Light mode	Dark mode	Token override*

*This was created by manually changing TwoLineTitleViewTokenSet to show UIColor.systemRed for the title and UIColor.systemOrange for the subtitle. This obviously isn't part of the change, but it's proof of concept that it works.

Pull request checklist

This PR has considered:

• Light and Dark appearances
• iOS supported versions (all major versions greater than or equal current target deployment version)
• VoiceOver and Keyboard Accessibility
• Internationalization and Right to Left layouts
• Different resolutions (1x, 2x, 3x)
• Size classes and window sizes (iPhone vs iPad, notched devices, multitasking, different window sizes, etc)
• iPad Pointer interaction
• SwiftUI consumption (validation or new demo scenarios needed)
• Objective-C exposure (provide it only if needed)

[laminesm]

Do these changes have any impact on Label token overrides?

[amgleitman]

Do these changes have any impact on Label token overrides?

Nope, Label token overrides still work just fine!