rustdoc: "copy path" icon should have a title attribute

[jsha]

To indicate what it does, i.e. "Copy use declaration to clipboard." Also, right now the action is triggered by an onclick= attribute in the HTML. It would be slightly nicer to use addEventListener in main.js, so it's easy to see in one place where all handlers are added.

[rukai]

Another observation to hopefully improve UX a bit, as I was very confused as to what this new feature did at first.
On crates.io the copy crate name icon looks like this:

Whereas the copy button looks like this:

Maybe its just how ⎘ displays on my machine?
But the crates.io copy is immediately obvious to me while with the docs copy I would need to look at the proposed title attribute to figure out whats going on.

[ChrisDenton]

crates.io also shows a message when you click the button that tells you it copied something to the clipboard:

[jsha]

Fixed!