Fix for Issue #4077- "brackets-shell: Can't build on Windows"

removes dependency on Microsoft SDK header file 'atlbase.h', which is not necessarily installed by all releases of Visual Studio.  Removed unnecessary use of ATL class CComPtr, and just managed the IShellItem pointer explicitly.

Also, removed a TODO comment pointed out in and made obsolete by PR# 247 (https://github.com/adobe/brackets-shell/pull/247).

Note: this change only affects the Windows implementation of brackets-shell.  Also, it doesn't change shell behavior at all.