Update UI for validation page and support tabbing

[AmelBawa-msft]

📖 Description

• Moved code from PreviewFileViewModel to PreviewSetViewModel
• Display error messages as an output tab for Get, Set and Test operatinos
• Added support for tabs in the validation page
• Other code enhancements

🎨 Update validation page

Before	After
	✅ Can cancel Get, Set and Test operations ✅ Validate multiple units in separate tabs ✅ Can update original source unit ✅ Display error messages in each tab 👇 Details below...

🏷️ Tabbing

✅ Validate multiple units in separate tabs

❌ Errors

✨ Output

Validate from configuration file

✅ Allow updating the source unit
✅ Allow returning back to the source set

🔗 References and Related Issues

🔍 How to Test

✅ Checklist

• Have you signed the Contributor License Agreement?
• Is there a linked Issue?
• Documentation updated?

[dkbennett]

Just a general suggestion to swap null checks to use "is null" and "is not null" for general readability and consistency, and protection against unexpected class behavior if the equals operator happens to be overloaded.