Show tooltip in wxTreeCtrl when item text doesn't fit on screen

This matches the behaviour of the native control in wxMSW and is
generally useful.

Closes https://github.com/wxWidgets/wxWidgets/pull/1397

include/wx/qt/private/treeitemdelegate.h

@@ -11,6 +11,7 @@
 #define _WX_QT_PRIVATE_TREEITEM_DELEGATE_H
 
 #include <QtWidgets/QStyledItemDelegate>
+#include <QtWidgets/QToolTip>
 
 #include "wx/app.h"
 #include "wx/textctrl.h"
@@ -67,6 +68,28 @@ public:
         QStyledItemDelegate::setModelData(editor, model, index);
     }
 
+    bool helpEvent(QHelpEvent *event, QAbstractItemView *view, const QStyleOptionViewItem &option, const QModelIndex &index)
+    {
+        if ( event->type() == QEvent::ToolTip )
+        {
+            const QRect &itemRect = view->visualRect(index);
+            const QSize &bestSize = sizeHint(option, index);
+            if ( itemRect.width() < bestSize.width() )
+            {
+                const QString &value = index.data(Qt::DisplayRole).toString();
+                QToolTip::showText(event->globalPos(), value, view);
+            }
+            else
+            {
+                QToolTip::hideText();
+            }
+
+            return true;
+        }
+
+        return QStyledItemDelegate::helpEvent(event, view, option, index);
+    }
+
 private:
     wxWindow* m_parent;
     mutable wxTextCtrl* m_textCtrl;