Revert "WLibrarySidebar: Remove #ifdef'd code for pre-Qt6 versions"

This reverts commit ea2d725.

As per the review comments in mixxxdj#13816 and the current policy in
https://github.com/mixxxdj/mixxx/wiki/Minimum-requirements-policy,
compatibility hacks should be kept unless removing them is necessary.

src/widget/wlibrarysidebar.cpp

@@ -66,7 +66,11 @@ void WLibrarySidebar::dragEnterEvent(QDragEnterEvent * event) {
 void WLibrarySidebar::dragMoveEvent(QDragMoveEvent * event) {
     //qDebug() << "dragMoveEvent" << event->mimeData()->formats();
     // Start a timer to auto-expand sections the user hovers on.
+#if QT_VERSION >= QT_VERSION_CHECK(6, 0, 0)
     QPoint pos = event->position().toPoint();
+#else
+    QPoint pos = event->pos();
+#endif
     QModelIndex index = indexAt(pos);
     if (m_hoverIndex != index) {
         m_expandTimer.stop();
@@ -89,7 +93,11 @@ void WLibrarySidebar::dragMoveEvent(QDragMoveEvent * event) {
             if (sidebarModel) {
                 accepted = false;
                 for (const QUrl& url : urls) {
+#if QT_VERSION >= QT_VERSION_CHECK(6, 0, 0)
                     QPoint pos = event->position().toPoint();
+#else
+                    QPoint pos = event->pos();
+#endif
                     QModelIndex destIndex = indexAt(pos);
                     if (sidebarModel->dragMoveAccept(destIndex, url)) {
                         // We only need one URL to be valid for us
@@ -146,9 +154,13 @@ void WLibrarySidebar::dropEvent(QDropEvent * event) {
             //eg. dragging a track from Windows Explorer onto the sidebar
             SidebarModel* sidebarModel = qobject_cast<SidebarModel*>(model());
             if (sidebarModel) {
+#if QT_VERSION >= QT_VERSION_CHECK(6, 0, 0)
                 QPoint pos = event->position().toPoint();
-                QModelIndex destIndex = indexAt(pos);
+#else
+                QPoint pos = event->pos();
+#endif
 
+                QModelIndex destIndex = indexAt(pos);
                 // event->source() will return NULL if something is dropped from
                 // a different application
                 const QList<QUrl> urls = event->mimeData()->urls();