Respect gadget aspect

Not sure this will be correct for more complex layout, it works now for mixer, will see later
rename a class to be consistent with the other
Correctly set default style on gadcon client class
Audio: middle-click on the speaker to toggle mute
Audio: show the volume channels in reverse order, from bottom to top
Audio: do not fail if the pulse connection cannot be made

Show a cannot-connect label in place of the sliders
Audio: retry the pulse connection every 5 secs
Audio: split mouse callbacks, to avoid listening unwanted event (like mouse,in)