add VariantType::Type to replace direct use of QMetaType::Type

[jkarneges]

#48320 mentions that working with variant types still involves QMetaType. This PR introduces our own substitute enum, with values that are identical to those of QMetaType, and updates the whole codebase to use it everywhere. This way, variant call sites don't have to reference QMetaType directly.

The enum looks like this:

#include <QMetaType>

namespace VariantType {
	enum Type {
		Invalid = QMetaType::UnknownType,
		Bool = QMetaType::Bool,
		Int = QMetaType::Int,
		Double = QMetaType::Double,
		LongLong = QMetaType::LongLong,
		String = QMetaType::QString,
		ByteArray = QMetaType::QByteArray,
		Hash = QMetaType::QVariantHash,
		Map = QMetaType::QVariantMap,
		List = QMetaType::QVariantList
	};
}

Since the values are the same, there should be no change in behavior.

Note that there is still some remaining direct usage of Qt's meta type system via the Q_DECLARE_METATYPE macro, which enables storing user-defined types in QVariant. We'll address that separately.