Get currentIndex from stackedWidget while closing

Earlier, we got the index from cursor's position this led to the close functionality to only work when cursor is hovered over tab title
We get the index straight from the stacked widget now.

src/tabbar.cpp

@@ -37,7 +37,7 @@ TabBar::TabBar(QWidget *parent) :
           });
     connect(app->getAction(KiwixApp::CloseTabAction), &QAction::triggered,
             this, [=]() {
-                auto index = this->tabAt(mapFromGlobal(QCursor::pos()));
+                auto index = currentIndex();
                 if (index < 0)
                     return;