[Win32] Use WS_EX_LAYERED style for native control holders with winui composition

[kekekeks]

Fixes #7106

[kekekeks]

Probably also fixes #6308

[adirh3]

@kekekeks after this change the DumbWindow's handle is never created (when using composition), causing some native controls to not work anymore since the parent handle is empty.
I checked it a bit and it seems like to create a layered child window you need to use specific manifest file - https://docs.microsoft.com/en-us/windows/win32/winmsg/using-windows
Works after doing so, but might want to somehow make it work from Avalonia or at least throw an exception

[maxkatz6]

Can we possibly detect if manifest is missing or not?
Otherwise, we can throw an exception if Handle is empty with information that manifest is required. It might be less useful, because window handle can be empty for many other reasons too.

I am not sure if it's the best idea if we force users to use manifest. I don't think other frameworks do that. Although we definitely can include it with templates.

[adirh3]

Can we possibly detect if manifest is missing or not? Otherwise, we can throw an exception if Handle is empty with information that manifest is required. It might be less useful, because window handle can be empty for many other reasons too.

I am not sure if it's the best idea if we force users to use manifest. I don't think other frameworks do that. Although we definitely can include it with templates.

If it's missing, then CreateWindowEx will fail which I think should be identifiable using GetLastError.
One thing we can do is fallback to non-layered window if layered one fails and have a warning log.

[kekekeks]

IMO we should just retry creating the window without WS_EX_LAYERED.