[UX] Layouts: Use icons within drop buttons on small/mobile screens

This came up in https://github.com/backdrop/backdrop-issues/issues/1935#issuecomment-225...

@olafgrabienski

... The motivation of my suggestion was the not so nice look of the User Login block title and it's Configure link in two rows, but I ain't got no better idea yet ...

@jenlampton

Yeah, I've scowled at that block several times too. We could certainly override the sidebar width for a few of these layouts to make the most-common ones look better, but it would be ideal to find a solution that works for all of them. Would you please open a new issue about this problem so don't forget to come up with better ideas? :)