moc: Use QMetaType::QVariant as the type for QVariant

moc: Use QMetaType::QVariant as the type for QVariant

QMetaType::QVariant has existed as a proper type for almost two
years, but the qvariant_nameToType function was written in 2006.

Using QMetaType::QVariant means QVariant can be treated just like
any other type. We can get rid of those hacky checks for LastType,
and the remaining checks become more readable.

The fact that QMetaProperty::{type,userType}() returned LastType
(0xffffffff) for QVariants was never documented (LastType itself is
internal). But there are other Qt modules that assume so. I'll fix
the ones I know about (qtdeclarative, qtscript, activeqt).

Change-Id: I799b9079bb8bbb1fe76c132525440b30415cbac5
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@nokia.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>