Perform save dumps before we pickle things.

This makes config.save_dump a powerful tool for debugging pickle
problems, where previously it was only useful for finding out the size
of save files.
sl2: Properly finish and imagemaps.

Previously, we were failing to finish the imagemaps we created, which
both leaked the md5 object and generally hurt performance.
Do not save ScreenDisplayable.children.

This is recreated on load, and saving it just leads to slower save and
load times.