Win32: Restore the versioned plugin installation path.

Win32: Restore the versioned plugin installation path.

Revert parts of g2ef72cb and g7710da4 so that the NSIS installer once
again places plugins in $INSTDIR\plugins\$VERSION. This matches the
behavior of Autotools and previous Windows installers, and reduces the
chances of a version mismatch if the user happens to install a custom
plugin.

Leave the development plugin path unversioned. Leave the extcap path
unversioned for now.

Change-Id: I861d4ee12975fba4b642e391871c5e852d92a2fe
Reviewed-on: https://code.wireshark.org/review/7976
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
Reviewed-by: Graham Bloice <graham.bloice@trihedral.com>
Petri-Dish: Graham Bloice <graham.bloice@trihedral.com>
Reviewed-by: Gerald Combs <gerald@wireshark.org>