MainWindow: Fix batch mode

[TryTwo]

I accidentally broke batch mode (for keeping the main window hidden) in #12794. Hopefully that's the only thing I missed, as I don't know another way to initialize the Window before it is initialized somewhat randomly with bugs on Windows platforms.

[TryTwo]

@AdmiralCurtiss Can I get this fix merged?

It uses the same check MainWindow::Show() already does. Also, getting positive feedback on the original commit from Windows users.

[Owlet7]

Using batch mode now resets the GUI window to the smallest size on the next time when Dolphin is run normally (on Windows).

[TryTwo]

Using batch mode now resets the GUI window to the smallest size on the next time when Dolphin is run normally (on Windows).

Is that the original behavior before I changed anything?

[Owlet7]

Is that the original behavior before I changed anything?

Prior to 5.0-21637, the main window size and position stayed the same after using batch mode.

[TryTwo]

Is that the original behavior before I changed anything?

Prior to 5.0-21637, the main window size and position stayed the same after using batch mode.

Oh okay. It's so fragile. Is there a way to bring the UI up in batch mode? I wonder if I can tell it to just not save new UI data if in batch.