Allow editing repeat scale via editor, and save this in map files.

I chose to add an extra XML tag storing only the needed data in order to
preserve compatibility with older versions that expect the specific
layout of the <SE k="..." /> tag, which can't be changed.
Adding a new tag like <SE m="..." /> with an updated layout is also
not an option, because older versions would not load these, and
maps would be empty as a result.