windows: custom 7zS.sfx for no installer UAC prompt, and icons

See 21e12c4 commit message for full explanation. Is there any objection to putting the customized 7zS.sfx in `contrib`? The file is about 180kB and should change infrequently.

If people don't want it in the repo then I will update the PR to store it somewhere else and download as part of `dist-extras`.