Add NotifcationManager property to TopLevel

[emmauss]

What does the pull request do?

Adds a NotificationManager property to TopLevel named NotificationManager, and change the host for WindowNotificationManager to TopLevel from Window. This allows managed notifications to be created through the TopLevel on all platforms.

What is the current behavior?

Managed notifications are only created using a WindowNotificationManager instance that requires a Window object. This prevents single view platforms from accessing it.

What is the updated/expected behavior with this PR?

How was the solution implemented (if it's not obvious)?

Checklist

• [ ] Added unit tests (if possible)?
• [ ] Added XML documentation to any related classes?
• [ ] Consider submitting a PR to https://github.com/AvaloniaUI/Documentation with user documentation

Breaking changes

Obsoletions / Deprecations

Fixed issues