Rollup merge of #83721 - GuillaumeGomez:copy-use, r=Nemo157

Rollup merge of #83721 - GuillaumeGomez:copy-use, r=Nemo157

Add a button to copy the "use statement"

Fixes https://github.com/rust-lang/rust/issues/50239

When clicking on the button, it'll add the elements prepended by "use " and will end with a ";". So in the images below, I now have in my clipboard `use std::fs::OpenOptions;`.

A screenshot of the newly added button:

![Screenshot from 2021-03-31 22-12-12](https://user-images.githubusercontent.com/3050060/113205430-90e64500-926e-11eb-8538-529829f611ec.png)

A screenshot after it was clicked:

![Screenshot from 2021-03-31 22-15-31](https://user-images.githubusercontent.com/3050060/113205532-ad827d00-926e-11eb-893d-35f2f8f92696.png)

r? `@Nemo157`