Make mime-send UI support optional.

* configure.ac (enable-mime-send): New option to enable mime sending.
* src/gpgoladdin.cpp (GpgolRibbonExtender::GetCustomUI): Add
 old UI again.

The idea is that Gpg4win can distribute an Addin without mime
send support (which currently also has more issues then reading)
before a version that only sends MIME messages.