Make DirectCompositionLayeredChildWindow compatible with Unicode build

[RatinCN]

The sample "DirectCompositionLayeredChildWindow" can compatible with Unicode build just make some very small changes.

And this PR also fix empty error message displayed due to

// Show error messages.
void CApplication::ShowErrorMessage(PCWSTR format, HRESULT hrErr)
{
    WCHAR msg[MAX_PATH];
    HRESULT hr = StringCbPrintf((STRSAFE_LPSTR)msg, sizeof(msg), (STRSAFE_LPCSTR)(L"%s (hr=0x%08X)"), format, hrErr);

    if (SUCCEEDED(hr))
    {
        MessageBox(NULL, (LPCSTR)msg, NULL, MB_ICONERROR);
    }
}

Where use %s to format a PCWSTR string even if in ANSI build.

Before:

After:

[zhuman]

I would typically just update projects to use L"" strings and only support Unicode but this is fine too :-)

[RatinCN]

@ChrisGuzak Okay to move on?