[UX] Change layout 'Visibility conditions' links to a drop button

As a follow-up to https://github.com/backdrop/backdrop-issues/issues/578, where we improved the display of layout's 'Visibility conditions', I'd like to once again suggest changing the condition links (configure & remove) to a drop button.

Not only would this provide consistency with the rest of Backdrop, but it would also fix the issue we apparently caused whereby the 'Remove' link comes before the 'Configure' one...