Fix main window turns blank after restoring from tray (#15031)

When restoring from tray icon, although the window manager shows qbt
window but qbt itself didn't handle the event correctly, i.e. the
`show()` was missing and thus qbt did nothing and the window is blank.
Note that at this point the `visible` property is `false`.
After invoking `show()` qbt will start showing the contents and also
fire another showEvent where `visible` property is `true` and here is where
qbt should handle preparations for the window.

Fix #9510.

src/gui/mainwindow.cpp

@@ -1131,17 +1131,26 @@ void MainWindow::on_actionStatistics_triggered()
 void MainWindow::showEvent(QShowEvent *e)
 {
     qDebug("** Show Event **");
+    e->accept();
 
-    if (currentTabWidget() == m_transferListWidget)
-        m_propertiesWidget->loadDynamicData();
+    if (isVisible())
+    {
+        // preparations before showing the window
 
-    e->accept();
+        if (currentTabWidget() == m_transferListWidget)
+            m_propertiesWidget->loadDynamicData();
 
-    // Make sure the window is initially centered
-    if (!m_posInitialized)
+        // Make sure the window is initially centered
+        if (!m_posInitialized)
+        {
+            move(Utils::Gui::screenCenter(this));
+            m_posInitialized = true;
+        }
+    }
+    else
     {
-        move(Utils::Gui::screenCenter(this));
-        m_posInitialized = true;
+        // to avoid blank screen when restoring from tray icon
+        show();
     }
 }