New SwiftUI DemoHostingController parent class

[mischreiber]

Platforms Impacted

• iOS
• visionOS
• macOS

Description of changes

Creating a base DemoHostingController, inheriting from FluentThemedHostingController, to centralize common logic for our SwiftUI demo controllers.

This is currently only used by ButtonDemoController_SwiftUI, but will expand to more going forward.

Binary change

n/a -- only affects demo app, not library

Verification

ButtonDemoController_SwiftUI still behaves as expected.

Visual Verification

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)

Microsoft Reviewers: Open in CodeFlow

[anandrajeswaran]

Probably should update PR description to explain the change (Description of changes section still has template text?)

[mischreiber]

Probably should update PR description to explain the change (Description of changes section still has template text?)

Whoops, not sure how that didn't get saved. Fixing!