Overhaul labels for encryption & signing / decrpytion & verification results

This is split out of !279 (merged) in order to keep the merge request at a manageable scope.

This roughly does the following:

• Improve the strings that are presented to the user:
  • Before this patch, the strings were mostly of the rather cryptic form "foo -> bar"
  • If the operation failed, the filename was also shown in a strikethrough style, which hardly seems understandable to a user and is apparently entirely unnoticable by a screenreader
• Some codepaths are cleaned up a bit
• Some edgecases are fixed (like showing the filename of the file that was s/mime verified when it was just decrypted)

GnuPG-Bug-ID: 6869