Add a copy button to the MarkdownPreview tool to copy the HTML output

[praneetloke]

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• 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?

There is no way to copy the HTML output from the markdown preview.

Issue Number: N/A

What is the new behavior?

A new copy button above the Webview control allows the user to copy the contents of the markdown-body element.

Screenshot

Other information

I am a newbie at UWP app development and especially with the MVVM pattern.

Quality check

Before creating this PR, have you:

• Followed the code style guideline as described in CONTRIBUTING.md
• Verified that the change work in Release build configuration
  • I could not get this to work on my machine for some reason even though my changes should not affect the Release build config. I was getting an error about a SetVersion task being unable to run because Microsoft.Build.Utilities version 3.5.0.0 wasn't available despite having a successful Nuget restore.
• Checked all unit tests pass

[praneetloke]

@veler I've updated the PR. Can you please take another look?

[veler]

Awesome! It looks all good :) Thank you for this!