Fixed glitch when dragging a movable QTabBar tab

When you would start to drag a tab, and then drag it in the other
direction along the point where you started the drag, the tab would fail
to follow the mouse for a distance twice the "startDragDistance", before
jumping to the mouse again.

Fixed this by only taking into account the "startDragDistance" when
checking whether a drag is started, and to rely on the "dragInProgress"
variable otherwise.

Change-Id: I5dd4ebd8340fef7e870f087be68c5097b49728cb
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/widgets/widgets/qtabbar.cpp

@@ -1869,9 +1869,8 @@ void QTabBar::mouseMoveEvent(QMouseEvent *event)
             }
         }
 
-        int offset = (event->pos() - d->dragStartPosition).manhattanLength();
         if (event->buttons() == Qt::LeftButton
-            && offset > QApplication::startDragDistance()
+            && d->dragInProgress
             && d->validIndex(d->pressedIndex)) {
             bool vertical = verticalTabs(d->shape);
             int dragDistance;