Utils: Introduce variantFromStore and storeFromVariant

These are functional replacements for
  QVariant::fromValue(QVariantMap) (or QVariant::fromValue(Store)) and
  QVariant::toMap() (or QVariant::toValue<Store>())

We will have a few code paths in the end that need to explicitly
operarate on both QVariantMap and Store (e.g. actual reading/writing
to keep format compatibility etc), so these can't in the end be
simple to/fromValue(OneType) but need an internal 'if' or such.

Change-Id: I954f3cb24fa8fe123162b72bbd25d891dd19b768
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>

src/libs/utils/persistentsettings.cpp

@@ -135,7 +135,7 @@ QVariant ParseValueStackEntry::value() const
     case QVariant::Invalid:
         return QVariant();
     case QVariant::Map:
-        return QVariant::fromValue(mapValue);
+        return variantFromStore(mapValue);
     case QVariant::List:
         return QVariant(listValue);
     default:
@@ -378,7 +378,7 @@ static void writeVariantValue(QXmlStreamWriter &w, const Context &ctx,
         w.writeAttribute(ctx.typeAttribute, QLatin1String(QVariant::typeToName(QVariant::Map)));
         if (!key.isEmpty())
             w.writeAttribute(ctx.keyAttribute, key);
-        const Store varMap = variant.value<Store>();
+        const Store varMap = storeFromVariant(variant);
         const Store::const_iterator cend = varMap.constEnd();
         for (Store::const_iterator i = varMap.constBegin(); i != cend; ++i)
             writeVariantValue(w, ctx, i.value(), i.key());