Handle multiple working directories

As described in #619 (closed) I tried to implement the handling of multiple working directories. The feature must be enabled via config only and old index files are not deleted (yet).

Unfortunately, only some tests did work in my environment (even though I think I followed all steps in the tutorial) because 'paperwork-cli chkdeps' did not succeed. But the index-tests succeeded.

Hope, this PR is still good enough to be accepted :)