Some tweaks for preview build

[veler]

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• New feature or enhancement
• UI change (please include screenshot!)
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Internationalization and localization
• Other (please describe):

What is the current behavior?

DevToys 2.0 only has a stable icon. There's no preview icon yet.

What is the new behavior?

By default, when building locally, DevToys has a "preview" icon.
The icon & splash screen is switched to "stable" or "preview" during publication. See https://github.com/DevToys-app/Publish/pull/3

Other information

Quality check

Before creating this PR:

• Did you follow the code style guideline as described in CONTRIBUTING.md
• Did you build the app and test your changes?
• Did you check for accessibility? On Windows, you can use Accessibility Insights for this.
• Did you verify that the change work in Release build configuration
• Did you verify that all unit tests pass
• If necessary and if possible, did you verify your changes on:
  • Windows
  • macOS
  • Linux