Security: Make NSIS only remove installed files.

• It's easy to get own files lost by installing to existing folder because for now we just remove entire destination folder, what is not best practice.
• It's also easy to delete e.g. documents or configs created by the user after installation of the app within the app's folder no matter where the folder is. Although config is separated we have no control over user's choice of folders used for stored files, especially on Windows.

Possible solution: http://nsis.sourceforge.net/Uninstall_only_installed_files

See also http://nsis.sourceforge.net/Talk:Uninstall_only_installed_files