xfce4-mixer has no session handler

[jp.guillemin]

Hi,

The xfce4-mixer window has no session support. I mean : if you launch it 
several times (ie : by double clicking on the mixer panel applet) you 
get 2 or more instances of the mixer simultaneously.

Obviously an application that has no reason to be loaded more than once 
should have session support to detect that an instance is already 
running and ignore repetitive launches.

JP