ReplayGainMode: add ToString(), FromString()

Move code from ReplayGainConfig.cxx.

src/ReplayGainMode.hxx

@@ -20,6 +20,8 @@
 #ifndef MPD_REPLAY_GAIN_MODE_HXX
 #define MPD_REPLAY_GAIN_MODE_HXX
 
+#include "Compiler.h"
+
 #include <stdint.h>
 
 enum class ReplayGainMode : uint8_t {
@@ -29,4 +31,19 @@ enum class ReplayGainMode : uint8_t {
 	AUTO,
 };
 
+/**
+ * Return the string representation of a #ReplayGainMode.
+ */
+gcc_pure
+const char *
+ToString(ReplayGainMode mode);
+
+/**
+ * Parse a string to a #ReplayGainMode.  Throws std::runtime_error on
+ * error.
+ */
+gcc_pure
+ReplayGainMode
+FromString(const char *s);
+
 #endif