Avalonia Import links

[ltrzesniewski]

This is the Avalonia version of #516 and #517:

There are a few caveats:

• AvaloniaEdit does not support underline, but there's a PR for this. I used bold in the meantime.
• AvaloniaEdit does not handle cursor changes very well (issue here, probably because of this), so I had to write some clumsy code to work around that.
• I only tested this on Windows.

I also fixed the display of Import/NoImport nodes in the tree (a space was missing), and made so that tabs can be closed with a middle-click.