chore: add embedded-resource for splashscreen definition

[Xiaoy312]

re: unoplatform/uno.toolkit.ui#1404

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature

What is the current behavior?

on mobile platforms, there is currently no way to retrieve the splash-screen image and background.

What is the new behavior?

an EmbeddedResource is now added that describes the screen-screen, which can be loaded by the app at runtime.

PR Checklist

Please check if your PR fulfills the following requirements:

• Tested code with current supported SDKs
• Docs have been added/updated which fit documentation template. (for bug fixes / features)
• Unit Tests and/or UI Tests for the changes have been added (for bug fixes / features) (if applicable)
• Wasm UI Tests are not showing unexpected any differences. Validate PR Screenshots Compare Test Run results.
• Contains NO breaking changes
• Updated the Release Notes
• Associated with an issue (GitHub or internal)

Other information

[welcome]

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