Added an nsis-based NVDA portable launcher, based on both the NVDA cd launcher and...

Added an nsis-based NVDA portable launcher, based on both the NVDA cd launcher and installer code.
This launcher is a single executable, which automatically self-extracts and runs a portable version of NVDA.
To build the launcher, for now run scons launcher.
Currently its very basic. Eventually  support for restarting NVDA, playing a sound while extracting could be added.
Eventually launcher will replace portable, and the installer will be built into NVDA removing the need for the NSIS installer.