The concept here is
PluginControl [not-yet-a SlavableAutomationControl] is-a AutomationControl
The AutomationControl base-class sets the baseline parameter-value.
This is also the value used for generic-control-UIs and Automation-Lane.
The PluginControl can modify this value when setting the actual
plugin control-parameter.
The Ardour-UI is not aware of the modulation and will have to explicitly
ask for the modulated value.
To custom plugin-UIs it will look as if the value is being automated.
The modulation value is to be set before running the plugin-instance
in connect_and_run().
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