Factor invokeMailer helper from ResultPage

Changes:
* Use QFileInfo instead of QString for the attachment parameter
* Leave showing a message box to inform the user about possible problems
with the attachment to the caller (because different text is needed for
the two callers).
* Use more appropriate text to inform the user when sending an OpenPGP
public key. In this case "save the request to a file" makes no sense and
there is nothing to drag because the Kleopatra icon is hidden if an
OpenPGP certificate is created.

GnuPG-bug-id: 5832