Rewrite as `Kleo::Commands::Command`.

See !16 (comment 427747)

Instead of using gpg-wks-binary on the commandline,
QGpgME::WKSPublishJob is now used, skipping creating, writing to,
reading from and finally deleting a temporary file on disk.
It also releases dependency on Libkleo to provide a path to the
gpg-wks-client binary.

Unlike the old version, successfully queueing a key publication request
mail does no longer produce an info box, a feature which might be
re-added.