Use Qt::DisplayRole when the lookup fails

This makes it do the right thing by default, and fixes filtering for
QStringListModels (and probably a few other ones as well).

declarativeimports/core/datamodel.cpp

@@ -60,7 +60,7 @@ void SortFilterModel::syncRoleNames()
 int SortFilterModel::roleNameToId(const QString &name)
 {
     if (!m_roleIds.contains(name)) {
-        return -1;
+        return Qt::DisplayRole;
     }
     return m_roleIds.value(name);
 }