GUI fix: show sdg save dialog again when user has chosen wrong file extension
better description for SDG files
prevent user from accidentally overwriting a sdg file; do not get stuck in loop if user wants to cancel
show save dialog at the beginning of loop
bugfix: avoid heap-eating infinite loop
add new setting for third-party classpath to SDG config
new SDG preparation module for joana.api
relax parameter type of ReducedCFGUtil.prepareForViewer