Improve startup window state handling

Replace current "Start qBittorrent minimized" option with "Initial window state" that allows to start qBittorrent as "hidden in system tray" while retaining regular "minimize to panel" functionality.

PR #18252.
Closes #487.

src/gui/mainwindow.h

@@ -1,5 +1,6 @@
 /*
  * Bittorrent Client using Qt and libtorrent.
+ * Copyright (C) 2022  Vladimir Golovnev <glassez@yandex.ru>
  * Copyright (C) 2006  Christophe Dumez <chris@qbittorrent.org>
  *
  * This program is free software; you can redistribute it and/or
@@ -35,6 +36,7 @@
 #include "base/logger.h"
 #include "base/settingvalue.h"
 #include "guiapplicationcomponent.h"
+#include "windowstate.h"
 
 class QCloseEvent;
 class QFileSystemWatcher;
@@ -74,13 +76,7 @@ class MainWindow final : public QMainWindow, public GUIApplicationComponent
     Q_DISABLE_COPY_MOVE(MainWindow)
 
 public:
-    enum State
-    {
-        Normal,
-        Minimized
-    };
-
-    explicit MainWindow(IGUIApplication *app, State initialState = Normal);
+    explicit MainWindow(IGUIApplication *app, WindowState initialState = WindowState::Normal);
     ~MainWindow() override;
 
     QWidget *currentTabWidget() const;