Improve copy functionality

[manuelwedler]

To make the copyable elements more easily recognizable, it would be nice if the cursor would change on token symbols, identicons, etc. A cursor: copy rule seems reasonable to me. Additionally changing the background color on hover as in the light client would be nice.

Furthermore we need some visual feedback that the copy was successful. We could show a little overlay like the network info on the top with a success message.