utils: add Mu::time_to_string

Helper function to format strings (a-la strftime).

lib/utils/mu-utils.hh

@@ -129,6 +129,19 @@ std::string date_to_time_t_string(const std::string& date, bool first);
  */
 std::string date_to_time_t_string(int64_t t);
 
+/**
+ * Get a string for a given time_t and format
+ * memory that must be freed after use.
+ *
+ * @param frm the format of the string (in strftime(3) format)
+ * @param t the time as time_t
+ * @param utc whether to display as UTC(if true) or local time
+ *
+ * @return a string representation of the time in UTF8-format, or empty in case
+ * of error.
+ */
+std::string time_to_string(const std::string& frm, time_t t, bool utc = false) G_GNUC_CONST;
+
 /**
  * Create a std::string by consuming a gchar* array; this takes ownership
  * of str which should no longer be used.