Fix the double border in windows with a search bar.

Merged Nikita Churaev requested to merge lamefun/gtk:searchbar-double-border-fix into gtk-3-22

Includes applications like GNOME Software and GNOME Documents. The search bar is a composite widget with a revealer inside it, and the content of the revealer is hidden, the border still remains. Changed the CSS to add style to the contents of the revealer instead of the search bar widget itself.

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