Proposal: Refactor `ITEMIDLIST` to a formal Structure and improve `IShellLink` type safety

[junkmd]

Description / Problem

Currently, the ITEMIDLIST definition in comtypes.shelllink is treated as a simple c_int.

comtypes/comtypes/shelllink.py

Lines 45 to 47 in 844a111

	# fake these...
	ITEMIDLIST = c_int
	LPITEMIDLIST = LPCITEMIDLIST = POINTER(ITEMIDLIST)

While this works as an opaque handle in some cases, it presents several issues for robust development:

1. Lack of Introspection: Developers cannot access internal fields of ITEMIDLIST and SHITEMID without manual casting, which is error-prone.
2. Incomplete Type Hinting: Methods such as GetIDList and SetIDList use hints.Incomplete, providing no useful information for IDEs or static analysis tools (like mypy).
3. Potential for Memory Errors: Without a formal structure, managing the memory of PIDLs is less intuitive.

Proposed Changes

• Formal Type Definitions:

  • Define SHITEMID and ITEMIDLIST as proper ctypes.Structure classes.
  • Ensure LPITEMIDLIST and LPCITEMIDLIST correctly pointer to these structures.

• Interface Enhancements:

  • Update IShellLinkA and IShellLinkW vtables to use these formal types.
  • Replace hints.Incomplete with _Pointer[ITEMIDLIST] and explicit HRESULT returns.

• Added Test Coverage:

  • Introduce unit tests in test_shelllink.py to verify round-tripping (Set/Get) of ITEMIDLIST.

Benefits

• Better autocompletion and type checking in modern IDEs.
• The library will more accurately reflect the underlying Windows API structure.
• Formal structures reduce the likelihood of pointer arithmetic errors or incorrect memory size assumptions.

Backward Compatibility

This change is expected to be highly backward compatible as LPITEMIDLIST remains a pointer.
Existing code treating it as a handle should continue to function without modification.