Reason why song importers are disabled is unclear

[Simon Hanna]

There are a couple of Song importers that are OS specific.
Mediashout for instance has the following string hardcoded
"... only supported on Windows. It has been disabled due to a missing python dependency ..."

The error confused me because I had the dependencies installed. I don't think programatically changing the error message is viable so I propose the following two options
1. Not show unsupported importers in the first place
2. Show all importers but gray out unsupported ones and disable their selection. Their name could be followed by a message like "unsupported on your platform"