Copy to clipboard with full color

[ltrzesniewski]

I was bored so I wrote this 😅

Fixes #3039

Problem

• Feature request: copy to clipboard with formatting #3039

  The text copied to the clipboard only had the basic syntax highlighting as defined in the xshd file. Additional highlighting provided by CSharpHighlightingTokenWriter wasn't present.

Solution

• Any comments on the approach taken, its consistency with surrounding code, etc.

  • I took the HTML copy code from AvalonEdit's Selection.CreateHtmlFragment as a baseline
  • The new code creates a HighlightedLine from the active RichTextModel
  • It then merges it into the one provided by the IHighlighter

• Which part of this PR is most in need of attention/improvement?

  The code is straightforward enough. I only wondered if this situation (having multiple DocumentColorizingTransformer instances) is common enough to warrant being handled by AvalonEdit itself, but that would require a rather large refactoring.

• At least one test covering the code changed

  No test; only the UI part changed.

[siegfriedpammer]

Thank you very much! :)