Fix NSIS template to more thoroughly use CPACK_PACKAGE_INSTALL_REGISTRY_KEY.
This allows different versions of software to have a separate sections in the
registry to keep track of things (installed components, and uninstall stuff).
Change default of CPACK_PACKAGE_INSTALL_REGISTRY_KEY to follow the value of
CPACK_PACKAGE_INSTALL_DIRECTORY so if an installation overwrites another installation,
the proper registry entries are more likely to be overwritten.
Fix CPack/NSIS generator to not insert code in the NSIS template to skip installation
of already installed components. This enables a repair like behavior and also enables
installing patch releases on top of an older installation.
e6d2bcfde2