Add user name and email to Insert > Fields

This is a low hanging fruit, since it's already available (although
quite hidden)

Generally, output "not set" i a pref is not set rather than an empty
string.

lib/ui/stdmenus.inc

@@ -431,6 +431,8 @@ Menuset
 		Item "Time (Fix)|x" "info-insert fixtime"
 		Separator
 		Item "File Name (Excl. Extension)|N" "info-insert buffer name-noext"
+		Item "User Name|U" "info-insert lyxrc user_name"
+		Item "User Email|E" "info-insert lyxrc user_email"
 		Separator
 		Item "Other...|O" "info-insert"
 	End

src/insets/InsetInfo.cpp

@@ -786,6 +786,9 @@ void InsetInfo::updateBuffer(ParIterator const & it, UpdateType utype) {
 		// remove \n and ""
 		result = rtrim(result, "\n");
 		result = trim(result, "\"");
+		gui = _("not set");
+		if (result.empty())
+			result = "not set";
 		setText(from_utf8(result), params_.lang);
 		break;
 	}