Use enum class for subtypes of CPDF_Annot.

Comparing CFX_ByteString for annotation subtypes is inefficient and
error-prone. This CL uses enum class to compare annotation subtypes.

Also, remove unused IPDFSDK_AnnotHandler::GetType() and
FSDK_XFAWIDGET_TYPENAME.

Review-Url: https://codereview.chromium.org/2295953002
diff --git a/fpdfsdk/cpdfsdk_annot.cpp b/fpdfsdk/cpdfsdk_annot.cpp
index 5ca439d..59d8f24 100644
--- a/fpdfsdk/cpdfsdk_annot.cpp
+++ b/fpdfsdk/cpdfsdk_annot.cpp
@@ -58,8 +58,8 @@
   return nullptr;
 }
 
-CFX_ByteString CPDFSDK_Annot::GetAnnotSubtype() const {
-  return "";
+CPDF_Annot::Subtype CPDFSDK_Annot::GetAnnotSubtype() const {
+  return CPDF_Annot::Subtype::UNKNOWN;
 }
 
 bool CPDFSDK_Annot::IsSignatureWidget() const {