Some small changes to the Windows NSI script

- on uninstall, delete all XSLT files and the "$instdir\xslt" folder itself
- manage a desktop icon (i believe we had that before?)
- ignore SVG files, as we are now embedding them as static resources

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>