Display widget signature

- but don't validate them for now;
  - Firefox will display a bar to warn that the signature validation is not supported (see https://bugzilla.mozilla.org/show_bug.cgi?id=854315)
  - almost all (all ?) pdf readers display signatures;
  - validation is done in edge but for now it's behind a pref.

src/shared/util.js

@@ -315,6 +315,7 @@ const UNSUPPORTED_FEATURES = {
   unknown: "unknown",
   forms: "forms",
   javaScript: "javaScript",
+  signatures: "signatures",
   smask: "smask",
   shadingPattern: "shadingPattern",
   /** @deprecated unused */