files-view: Use "suggested-action" style class when showing "No Results Found"

When showing "No Results Found", the "Search Settings" button can help the user to fix the problem of no relevant results showing. Add the "suggested-action" style class to the button to draw the attention of the user.

"Suggested action highlights a button for affirmative action. This can be used to draw attention to the next step in a process or a call to action."

https://developer.gnome.org/hig/patterns/controls/buttons.html#buttons