Better copying support

[Alizter]

This issue is a collection of related issues about copying.

1. The only way to copy selected text in the window is using Ctrl+C. Together with vim mode, this can be inconvenient. I think having a copy button would be useful.

2. The structural parts of the goal window cannot be selected. Things like the hypothesis colons, proof bar etc.

3. The right click copy menu is useless.

[ejgallego]

Thanks to @Alizter for bringing this issue to my attention, a couple of comments:

• having a copy button would be great, that should be some basic HTML + JS programming, if in doubt, the way lean4-vscode does it is a good reference
• making the html into something that is rendered properly can vary in difficulty:
  • I'm not sure how selecting with the mouse can work
  • on the other hand, writing a small JS function that renders in text from the GoalsAnswer<String> provided in the view would be trivial, so the copy button could call that function before copying indeed (and even query the open/close status of some UI items)