Midi: no virtual ports on Windows

Dear Martin,

On Windows, RtMidi does not implement the virtual port feature (there are numerous alternatives for virtual midi ports, I personally use "loopmidi"). This PR disables the controls in Joker and in MidiTool on platforms where it is not available.

It also cleans up a little bit the logic for selecting either the virtual or real port. The current code could insert an existing port name that no longer exists in the virtual port setting, and I think it could be disturbing for a user...

Thanks again for your work on Joker ! MTC is very interesting !

Best regards,

Timothée