CPack/NSIS: Fix reinstall and multiple install issues when using components.

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.

Source/CPack/cmCPackNSISGenerator.cxx

@@ -672,9 +672,6 @@ CreateComponentDescription(cmCPackComponent *component,
     }
   componentCode += "  SetOutPath \"$INSTDIR\"\n";
 
-  componentCode += "  IntCmp $" + component->Name 
-    + "_was_installed ${SF_SELECTED} noinstall_" + component->Name + "\n";
-
   // Create the actual installation commands
   if (component->IsDownloaded)
     {
@@ -824,7 +821,6 @@ CreateComponentDescription(cmCPackComponent *component,
     componentCode += "  File /r \"${INST_DIR}\\" +
       component->Name + "\\*.*\"\n";
     }
-  componentCode += "  noinstall_" + component->Name + ":\n";
   componentCode += "SectionEnd\n";
 
   // Macro used to remove the component