QGridLayoutEngine: print stringified layout item in warnings

This provides a way for the user to identify any items involved in
warnings we may print, as e.g. their objectName will be included.

Qt Quick's Layout types can use this to stringify the QQuickItem,
which will result in the type and objectName being printed.

Change-Id: I3d6b6ea0315df98629992afd29e4bf2aad847e22
Reviewed-by: Jan Arve Sæther <jan-arve.saether@qt.io>

src/gui/util/qgridlayoutengine.cpp

@@ -964,8 +964,11 @@ void QGridLayoutEngine::insertItem(QGridLayoutItem *item, int index)
 
     for (int i = item->firstRow(); i <= item->lastRow(); ++i) {
         for (int j = item->firstColumn(); j <= item->lastColumn(); ++j) {
-            if (itemAt(i, j))
-                qWarning("QGridLayoutEngine::addItem: Cell (%d, %d) already taken", i, j);
+            const auto existingItem = itemAt(i, j);
+            if (existingItem) {
+                qWarning("QGridLayoutEngine::addItem: Can't add %s at cell (%d, %d) because it's already taken by %s",
+                    qPrintable(item->toString()), i, j, qPrintable(existingItem->toString()));
+            }
             setItemAt(i, j, item);
         }
     }

src/gui/util/qgridlayoutengine_p.h

@@ -24,6 +24,7 @@
 #include <QtCore/qpair.h>
 #include <QtCore/qsize.h>
 #include <QtCore/qrect.h>
+#include <QtCore/qdebug.h>
 
 #include <float.h>
 #include "qlayoutpolicy_p.h"
@@ -283,6 +284,8 @@ public:
 
     virtual QLayoutPolicy::ControlTypes controlTypes(LayoutSide side) const;
 
+    inline virtual QString toString() const { return QDebug::toString(this); }
+
     QRectF geometryWithin(qreal x, qreal y, qreal width, qreal height, qreal rowDescent, Qt::Alignment align, bool snapToPixelGrid) const;
     QGridLayoutBox box(Qt::Orientation orientation, bool snapToPixelGrid, qreal constraint = -1.0) const;