Change some RString methods to free functions

These ones aren't a std::string method. Doing this helps the RString to
std::string migration.

src/PercentageDisplay.cpp

@@ -184,7 +184,7 @@ void PercentageDisplay::Refresh()
 			}
 
 			// HACK: Use the last frame in the numbers texture as '-'
-			sNumToDisplay.Replace('-','x');
+			Replace(sNumToDisplay, '-', 'x');
 		}
 	}