Warn on private inner classes

[ivaynberg]

if a property is inside a private inner class it will not be visible to anyone, eg

 public class Foo {
   private static class Bar {
      @Property int baz;
   }
 }

Then BarMeta has to contain a reference to Foo.Bar.baz which it cannot because Bar is private.

When this happens we should either ERROR or WARN if @Property is used and not generate the binding if @Bean or a like general class annotation is used.