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: T5832