mixer: lock up the slider for the case a drag is in progress
mixer: lock up the slider for the case a drag is in progress

if you click on the slider in the gadget the slider jumps a back to the
old volume and then to the new volume, this is because a pulseaudio
update is sent while the last changed event was called, but not the
drag_stop callback.

tasn/enlightenment,tasn/enlightenment,tasn/enlightenment