Implement "shutting-down shell" functionality

Implement "shutting-down shell" functionality without need for killing explorer.exe instances in the taskmgr: see https://msdn.microsoft.com/en-us/library/windows/desktop/cc144064(v=vs.85).aspx for more details.

Note that the method described in "For Windows XP:" list point still works under Windows 10.
It might be useful to add some more available option, for example as a menu item in the start-menu context popup menu.

Cc Smiley or gigaherz, if you want to have something easy to code in the shell.