UI-Coordinates: fix insidious bug

util::contains used to pick the overload for strings,
i.e. it first converted the UI-Coordinates to diagnostic output format
and then searched that string for '*' to determine if the pattern is explicit

works as expected, but not what you'd intend....
...and breaks spectacularly once you search for something as innocuous as '.'

src/gui/interact/ui-coord.hpp

@@ -290,6 +290,7 @@ namespace interact {
       
       /* === String representation === */
       
+      explicit
       operator string()  const
         {
           if (isnil (*this))