DecryptVerifyFilesController: fix verification of opaquely signed messages

• gpgme_op_decrypt_verify only works for opaquely signed messages when protocol==OpenPGP, so we now classify the input file in the DecryptVerifyOpaque case and only use DecryptVerifyTask if the input is potential ciphertext, otherwise we use VerifyOpaqueTask.

Partly fixes kolab/issue4156 (gpg4win).
What's missing is either bullet-proof classification
(also for binary content) or a way for the user to select decrypt
over verify-opaque in the UI.

svn path=/branches/kdepim/enterprise4/kdepim/; revision=1095058