QRMA: unconstify root index member for tree adapters

While only set during construction, making it const breaks assignment.

Add a test case that implement a search algorithm that finds a matching
row within a tree hierarchy. For that to work (withing using an
expensive recursive implementation) we need an assignment operator.

Change-Id: I52c3504acbd9a5196777c952e69b177de9abcddd
Reviewed-by: Artem Dyomin <artem.dyomin@qt.io>
(cherry picked from commit 4d7dfcf)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

Author: vohi

src/corelib/itemmodels/qrangemodeladapter_impl.h

@@ -361,7 +361,7 @@ struct ParentIndex
 template <>
 struct ParentIndex<true>
 {
-    const QModelIndex m_rootIndex;
+    QModelIndex m_rootIndex;
     QModelIndex root() const { return m_rootIndex; }
 };
 

tests/auto/corelib/itemmodels/qrangemodeladapter/tst_qrangemodeladapter.cpp

@@ -77,6 +77,7 @@ private slots:
     void treeIterate();
     void treeAccess();
     void treeWriteAccess();
+    void treeFindRecursively();
 
     void insertRow();
     void insertRows();
@@ -2038,6 +2039,29 @@ void tst_QRangeModelAdapter::treeWriteAccess()
     }
 }
 
+void tst_QRangeModelAdapter::treeFindRecursively()
+{
+    QRangeModelAdapter adapter(createValueTree());
+    QStringView needle = u"2.3.2";
+    QList<int> path;
+    while (true) {
+        auto it = std::find_if(adapter.cbegin(), adapter.cend(), [needle](const auto &row){
+            return needle.startsWith(row->value());
+        });
+        QVERIFY(it != adapter.cend());
+        path.append(it - adapter.cbegin());
+        if (it->value() == needle)
+            break;
+        if ((*it).hasChildren()) {
+            adapter = (*it).children();
+        } else {
+            path = {};
+            break;
+        }
+    }
+    QCOMPARE(path, (QList<int>{1, 2, 1}));
+}
+
 void tst_QRangeModelAdapter::insertRow()
 {
     {