Core: Namespace NetPlay utilities under the NetPlay namespace

Previously there was only one function under the NetPlay namespace,
which is kind of silly considering we have all of these other types
and functions existing outside of the namespace.

This moves the rest of them into the namespace.

This gets some general names, like Player, for example, out of the global namespace.

Source/Core/Core/NetPlayClient.h

@@ -18,6 +18,8 @@
 #include "Core/NetPlayProto.h"
 #include "InputCommon/GCPadStatus.h"
 
+namespace NetPlay
+{
 class NetPlayUI
 {
 public:
@@ -186,3 +188,4 @@ private:
 
 void NetPlay_Enable(NetPlayClient* const np);
 void NetPlay_Disable();
+}  // namespace NetPlay