Replace MainWindow with plain Window in sample projects

[AndrewKeepCoding]

Description

This PR updates WindowHelper.CreateWindow() to return a plain Window instead of MainWindow, which was tightly coupled to several static members.

Motivation and Context

Fixes #2062.

How Has This Been Tested?

Manually tested.

Screenshots (if appropriate):

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)