Rework file decryption

If the decryption fails the filenames of those files are shown.
The error message given in this case is not perfect, but because we are in string freeze I reused one that already existed.

Before this for every file to decrypt one KgpgLibrary object was created and never deleted again.

Now only one object is created for every decryption job (which may contain several files at once) and afterwards is properly destroyed.