Add support for QFlags in QMetaObject & QMetaType

QMetaType currently does not know about QFlags, i.e. none of the QMetaType::TypeFlags indicate it and MetaType::IsEnumeration is not set.

The latter is a little inconsistent, because if the QFlags are declared with the Q_FLAG macro, you can perfectly well get the underlying QMetaEnum with QMetaEnum::fromType() and the flags are listed as enumerators in the QMetaObject of the containing type.

My suggestion would be to either set QMetaType::IsEnumeration for QFlags or to introduce a QMetaType::IsFlags type flag.

This bug has occured to us in QtWebChannel development when trying to marshall QFlags to integers and vice-versa. The current way of detecting QFlags is rather expensive by going through the enumerators of the containing QMetaObject by name.

Then the next problem occurs: QMetaType should probably have converter functions for int -> QFlags and QFlags -> int, which it currently doesn't have.

All in all this makes working with QMetaType pretty ugly as soon as QFlags types are involved.