OFNOTIFY metadata should be unified

[JeremyKuhne]

Currently we have:

[SupportedArchitecture(Architecture.X64 | Architecture.Arm64)]
public struct OFNOTIFYW
{
	public NMHDR hdr;
	public unsafe OPENFILENAMEW* lpOFN;
	public PWSTR pszFile;
}

[StructLayout(LayoutKind.Sequential, Pack = 1)]
[SupportedArchitecture(Windows.Win32.Interop.Architecture.X86)]
public struct OFNOTIFYW
{
	public NMHDR hdr;
	public unsafe OPENFILENAMEW* lpOFN;
	public PWSTR pszFile;
}

public struct NMHDR
{
	public HWND hwndFrom;
	public UIntPtr idFrom;
	public uint code;
}

Packing of 1 doesn't actually change the layout as there are no field gaps on 32 bit. Same goes for OFNOTIFYA, OFNOTIFYEXA, and OFNOTIFYEXW. This is preventing Windows Forms from consuming this type (through CSWin32) for our build (which is AnyCPU).

[mikebattista]

Related issue #613.

[mikebattista]

See #1300 (comment).