Grey out the "Start" button in the Pulseaudio bridge if the pulseaudio JACK module is not installed

[fpesari]

Hello,

a user reporter that he was unable to start the Pulseaudio bridge from Cadence because the module failed to load. This was because he didn't have the right package (pulseaudio-module-jack on openSUSE) installed. But IMHO the right behavior would have been to prevent them from clicking the button by greying it out, or by not showing the Pulseaudio tab at all.