LibCore: Put all classes in the Core namespace and remove the leading C

I've been wanting to do this for a long time. It's time we start being
consistent about how this stuff works.

The new convention is:

- "LibFoo" is a userspace library that provides the "Foo" namespace.

That's it :^) This was pretty tedious to convert and I didn't even
start on LibGUI yet. But it's coming up next.

Servers/ProtocolServer/PSClientConnection.h

@@ -40,7 +40,7 @@ class PSClientConnection final : public IClientConnection<ProtocolServerEndpoint
     , public ProtocolServerEndpoint {
     C_OBJECT(PSClientConnection)
 public:
-    explicit PSClientConnection(CLocalSocket&, int client_id);
+    explicit PSClientConnection(Core::LocalSocket&, int client_id);
     ~PSClientConnection() override;
 
     virtual void die() override;