fix(ui): Set Qt::Window flag in multi windows mode

This sets the flag to `Qt::Window` for open dialogs in multi windows mode.
Commit changes the appearance of non-chat windows (Settings, Add Friend, ...).

src/widget/widget.cpp

@@ -1515,7 +1515,7 @@ ContentLayout* Widget::createContentDialog(DialogType type) const
     {
     public:
         explicit Dialog(DialogType type)
-            : ActivateDialog()
+            : ActivateDialog(nullptr, Qt::Window)
             , type(type)
         {
             restoreGeometry(Settings::getInstance().getDialogSettingsGeometry());