Process Panner Automatables (emit Changed)

Process Panner Automatables (emit Changed)

Pan-automation is evaluated directly from the control-list.
::distribute_automated() does not update the controls.

However, the *owner* of each automation-control is responsible to
evaluate automation of automated automation-controls (and emit Changed()
signals to notify the GUI and slaved controls).

This follow the same concept as PluginInsert: The Changed signal
is called on demand when evaluating automation.

This fixes pan-automation-sliders (automation-lane header) not updating.