theme/entries: Set entry hint-text color

Set the color for hint-texts in entries to a more transparent value so
it's clear that this is a hint, not the actual text inside the entry.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/944

data/theme/gnome-shell-sass/widgets/_entries.scss

@@ -20,4 +20,5 @@ StEntry {
     icon-size: $base_icon_size;
     padding: 0 4px;
   }
-}
+  StLabel.hint-text { color: transparentize($fg_color, 0.3); }
+}