QLineEdit: Create a separate undo entry for each middle-click paste

Previously, a sequence of middle-click pastes produced just one undo
item to undo/redo all of the pastes at once.
The new code seems to be the intended way to paste the selection anyway.

Change-Id: Ifc2e9714628da9e394053ff8c34709578656f54d
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/widgets/widgets/qlineedit.cpp

@@ -1582,7 +1582,7 @@ void QLineEdit::mouseReleaseEvent(QMouseEvent* e)
             d->control->copy(QClipboard::Selection);
         } else if (!d->control->isReadOnly() && e->button() == Qt::MidButton) {
             deselect();
-            insert(QApplication::clipboard()->text(QClipboard::Selection));
+            d->control->paste(QClipboard::Selection);
         }
     }
 #endif