AK: Really disallow making OwnPtrs from refcounted types

This looks at three things:
- if the type has a typedef `AllowOwnPtr', respect that
- if not, disallow construction if both of `ref()' and `unref()' are
  present.
Note that in the second case, if a type only defines `ref()' or only
defines `unref()', an OwnPtr can be created, as a RefPtr of that type
would be ill-formed.

Also marks a `Performance' to explicitly allow OwnPtrs.

AK/RefCounted.h

@@ -65,6 +65,7 @@ class RefCountedBase {
 
 public:
     typedef unsigned int RefCountType;
+    using AllowOwnPtr = FalseType;
 
     ALWAYS_INLINE void ref() const
     {