QMetaType ignores overridden memory allocators

QMetaType ignores overridden memory allocators when creating objects.
This is very annoying, as it does not allow to use QVariable and Qt Property System for objects (or classes with fields) that must be aligned in memory (e.g. Eigen classes that use SSE).

I found that it was broken in commit 3d575d4 by replacing `new T` to `new(operator new(size)) T` (Create to Construct at "new operator(size)").

It turns 8 byte alignment instead of the 16 in my case, which could not be altered.