protocol/ArgParser: move struct RangeArg to separate header

src/protocol/ArgParser.hxx

@@ -22,10 +22,9 @@
 
 #include "check.h"
 
-#include <limits>
-
 #include <stdint.h>
 
+struct RangeArg;
 class SongTime;
 class SignedSongTime;
 
@@ -38,19 +37,6 @@ ParseCommandArgInt(const char *s, int min_value, int max_value);
 int
 ParseCommandArgInt(const char *s);
 
-struct RangeArg {
-	unsigned start, end;
-
-	void SetAll() {
-		start = 0;
-		end = std::numeric_limits<unsigned>::max();
-	}
-
-	static constexpr RangeArg All() {
-		return { 0, std::numeric_limits<unsigned>::max() };
-	}
-};
-
 RangeArg
 ParseCommandArgRange(const char *s);