Add infrastructure for making Toucan available as a regular application.

The build-download directory contains all necessary files to build
Toucan download archives.

First generate the toucan/toucan.exe by running launch4j with
toucan-client-launch4j.xml as configuration file.

Then run ./build_toucan_download_files.sh to build:
  - toucan.zip
  - toucan.tar.gz

Signed-off-by: Gert Hulselmans <gert.hulselmans@med.kuleuven.be>