Add a quick way to add a reference to a dub project

[Orvid]

Assuming that this ends up being implemented, then it should become possible to create a nice add-reference window for dub projects.

As it is, it would be useful even if we had to type in the name and version of the package manually.

[aBothe]

Well okay, dub list-available or so might make sense. I just don't want to write stuff into the dub.json due to properly formatting all the stuff and so on :/ -- too lazy to do that.