Added WKS publishing support.

Some mail providers offer GnuPG WebKey Service, publishing a user's
public PGP key on the provider's WebKey Directory using a well-defined
string of mails to verify the submitter is actually the owner of the key
and approves of the submission, rectifying some of the shortcomings of
public key servers.

The process is defined in
https://www.ietf.org/archive/id/draft-koch-openpgp-webkey-service-13.html

Justification: Thunderbird's engimail extension already supports
publishing public PGP keys using the same process and KMail is already
capable of handling the authorization request and response mails from
the provider, at which point Kleopatra is no longer involved, Kleopatra
just needs to create and send the initial publication request mail with
the key's user id as the sender address, which is a feature this patch
adds.

Functional description: This patch adds a context menu entry for PGP
keys for which a secret key is in the keyring to "Publish at mail
provider". Calling this entry will try to publish the key at the key's
first user id's mail provider. Likewise it adds a context menu entry for
each such key's user id which provides the option to select which user
id should be used to determine the mail provider and sending user in
question.
Provided the user continues the publishing process after a warning
message, gpg-wks-client is called with the selected user id which then
figures if the mail provider supports publishing via WKS in the first
place and if so, will produce a temporary file with the mail to be sent.
If gpg-wks-client is unable to find the necessary submission mail address
and PGP key for the user id's mail provider, an error message is shown
and the process aborted.
Otherwise Kleopatra invokes Akonadi to select the user id's mail
transport and will use it to send the previosuly created mail. If no
such transport can be found, the process is aborted.
When the mail has been handed over to Akonadi mail transport,
Kleopatra's part in publishing the PGP key at the mail provider is
complete, the rest is handled by the user's mail agent.