Slicer3:Extensions:Directories

TemporaryDirectory is user-defined via Application Settings, defaults to /Users/$USER/Slicer3$USER

ExtensionInstallPath is user-defined via Application Settings, defaults to value of TemporaryDirectory if not set by user

* Zip files downloaded to TemporaryDirectory
* Extensions installed to Slicer3 SVN Revision-based subdirectory of ExtensionInstallPath (default: /Users/$USER/Slicer3$USER/@SVN@)
* Slicer3 "default" extensions in Slicer3_INSTALL_MODULES_LIB_DIR