Add context menu with copy action in preview markdown files

[sophiali23]

Feature Description:

No context menu is rendered when I right-click in a preview markdown file. It would be nice to have one where I can perform actions like copy.

[tsmaeder]

There are actually two markdown previews available in Theia IDE: one is contributed by the "preview" package and one is a webview coming from the VS Code built-in. While we have no control over the VS Code built-in, we could do sth. about our own package.
I don't see any useful commands in a preview though beyond "copy". Note that the 'ctrl-c' shurtcut works in the preview.