flags: rename flags_to_string -> to_string

For consistency.

lib/message/mu-flags.cc

@@ -27,7 +27,7 @@
 using namespace Mu;
 
 std::string
-Mu::flags_to_string(Flags flags)
+Mu::to_string(Flags flags)
 {
 	std::string str;
 

lib/message/mu-flags.hh

@@ -328,7 +328,7 @@ flags_filter(Flags flags, MessageFlagCategory cat)
  *
  * @return string as a sequence of message-flag shortcuts
  */
-std::string flags_to_string(Flags flags);
+std::string to_string(Flags flags);
 
 } // namespace Mu