Add win32 support for listing windows

[clemux]

Listing windows seems to be working correctly. However :

• capturing does not work for all windows. I get a black or white screen for some of them.
• I don't know what xcffib.xproto.MapState.Viewable means. If it is equivalent to https://learn.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-iswindowvisible, I'll have to change _enum_windows_cb. For now it always returns (window_name, True)
• I haven't tested on X11 to check that I didn't break anything
• Tested only on Windows 11

I'l leave this PR a draft for now.

[ubitux]

Nice, thanks.

It seems you didn't break X11, works fine here, so feel free to remove the draft.

[ubitux]

Listing windows seems to be working correctly. However :

* capturing does not work for all windows. I get a black or white screen for some of them.

Does that happen for viewable windows?

* I don't know what `xcffib.xproto.MapState.Viewable` means. If it is equivalent to https://learn.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-iswindowvisible, I'll have to change `_enum_windows_cb`. For now it always returns `(window_name, True)`

It's supposed to be, I guess.

I will make it obvious in the combo box which window is visible or not (italic for non-visible, or maybe bold for visible)

Ah, also, can you update the README for the Windows mention?