Add QML_PROTOTYPE macro to explicitly override metatype base class detection

When generating metatypes, only the direct base classes are picked, which is a problem for Language Server if we wrap around a templated class, as following

class Base : public QObject {
Q_OBJECT
...
};

template<typename T>
class TBase : public Base {
...
};

class Derived : public TBase<Item> {
Q_OBJECT
...
};

At the moment, Qt would detect `TBase<Item>` as a base class. It doesn't give Language Server enough information to provide useful diagnostics. Instead, we could directly specify indirect base class.

class Derived : public TBase<Item> {
Q_OBJECT
QML_PROTOTYPE(Base)
...
};