Hide Property<T> and PropertyTracker<T> from the public C++ API

Move those two classes into the private_api namespace, which is excluded
from the API reference documentation.

For generate code the explicit qualification of Property<T> is changed,
for the cbindgen generated item types the private_api namespace is
pulled into the cbindgen_private namespace.

api/sixtyfps-cpp/include/sixtyfps_properties.h

@@ -65,8 +65,6 @@ inline void sixtyfps_property_set_animated_binding_helper(
             handle, binding, user_data, drop_user_data, animation_data, transition_data);
 }
 
-}
-
 template<typename T>
 struct Property
 {
@@ -303,4 +301,6 @@ private:
     cbindgen_private::PropertyTrackerOpaque inner;
 };
 
+} // namespace private_api
+
 } // namespace sixtyfps