Context menu for comment threads

[laurentlb]

From @alexr00 (in #150824):

Finally, for comments it might be nice to have all these actions in a context menu on the whole comment peek view. I don't think I'm the only one who will right click on things to see if there are some actions I don't know about.

I think that's a simple feature to add:

1. Add a 3-dot menu on the comment thread title (at the right of other contributed icons)
2. Support right-clicking on the comment thread title

The combination of 3-dot + right-click already exists in other parts of vscode and would improve the experience on wide screens.

This would also help us rely less on icons. New users are often confused with all the icons. Plus, there are multiple features we didn't expose precisely because they cannot be explained with an icon.

cc @hermannloose

[alexr00]

Add a 3-dot menu on the comment thread title (at the right of other contributed icons)

This one may actually be possible for extensions to do with submenus. I'll double check.

[alexr00]

Optimistically adding for June, but will likely end up moving to July.