QTreeView: make autoExpand feature usable, it should never auto-collapse

The code would toggle between expanded and collapsed every time the
mouse moves over the item (after the autoExpandDelay), which was unusable.
As reporters point out (and I fully agree), collapsing the item under
the mouse is never wanted.

I attempted an auto-collapse when moving out of the current subtree,
but that was not usable either: when moving the mouse down, the auto-collapsing
of a previous sibling means that the item we're targeting has suddenly jumped
up, it's a frustrating experience trying to aim for a moving target.

The only behavior that is usable is the most simple one: auto expand,
never auto collapse.

Fixes: QTBUG-2553
Pick-to: 6.8
Change-Id: I2c56dd668b2df4ec23bbbe829da6e8cdd049f376
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>
Reviewed-by: Christian Ehrlicher <ch.ehrlicher@gmx.de>

src/widgets/itemviews/qtreeview.cpp

@@ -310,7 +310,7 @@ void QTreeView::setHeader(QHeaderView *header)
   \brief The delay time before items in a tree are opened during a drag and drop operation.
 
   This property holds the amount of time in milliseconds that the user must wait over
-  a node before that node will automatically open or close.  If the time is
+  a node before that node will automatically open.  If the time is
   set to less then 0 then it will not be activated.
 
   By default, this property has a value of -1, meaning that auto-expansion is disabled.
@@ -1266,7 +1266,7 @@ void QTreeView::timerEvent(QTimerEvent *event)
         if (state() == QAbstractItemView::DraggingState
             && d->viewport->rect().contains(pos)) {
             QModelIndex index = indexAt(pos);
-            setExpanded(index, !isExpanded(index));
+            expand(index);
         }
         d->openTimer.stop();
     }