[Linux] Fix minimize to tray (silent start) option

[proninyaroslav]

According to the window_manager documentation, to start a window in hidden mode, we need to use gtk_widget_realize instead of gtk_widget_show to create the window widget. But since Flutter 3.24, this is broken (I checked and it is indeed broken) leanflutter/window_manager#179 (comment). The solution is to call gtk_widget_hide immediately after calling fl_register_plugins. In this case, the window is correctly minimized. I demonstrated the window operation with and without the minimize to tray option on the video (build and run steps are cut off).

screencast.mp4

Close #1437