Store the filter string in the member variable

Doh.

Change-Id: I529e79e947614545a7bc9f1d4e4a2191e238feaa

src/declarativeimports/core/datamodel.cpp

@@ -109,6 +109,7 @@ void SortFilterModel::setFilterString(const QString &filterString)
     if (filterString == m_filterString) {
         return;
     }
+    m_filterString = filterString;
     QSortFilterProxyModel::setFilterFixedString(filterString);
     filterStringChanged(filterString);
 }