Add an utility function to render signed percentage.

Using Unicode minus and +0% convention (to indicate that it's not an
"empty" 0%)

src/serialization/string_utils.cpp

@@ -249,6 +249,14 @@ bool string_bool(const std::string& str, bool def) {
 	return true;
 }
 
+std::string signed_percent(int val)
+{
+	std::ostringstream oss;
+	//NOTE: "−" is the Unicode minus
+	oss << (val >= 0 ? "+" : "−") << abs(val) << "%";
+	return oss.str();
+}
+
 static bool is_username_char(char c) {
 	return ((c == '_') || (c == '-'));
 }

src/serialization/string_utils.hpp

@@ -129,6 +129,9 @@ std::string &strip(std::string &str);
 /** Convert no, false, off, 0, 0.0 to false, empty to def, and others to true */
 bool string_bool(const std::string& str,bool def=false);
 
+/** Convert number into percentage (using the Unicode "−" and +0% convention */
+std::string signed_percent(int val);
+
 /**
  * Try to complete the last word of 'text' with the 'wordlist'.
  *