GenericUI: Update all input controls on preset load

GenericUI: Update all input controls on preset load

The existing code relies on AutomationControls for getting parameter
changes and update the UI accordingly. One case where this doesn't yet
work is preset loading, where ARDOUR::Plugin is responsible for actually
loading the preset but doesn't notify the changes to AutomationControls.

Since the input_controls vector now contains all ControlUI's that rely on
AutomationControls to get updates, just listen to Plugin::PresetLoaded()
and trigger an update of all elements in input_controls.

This is temporary until a better solution is devised to make
AutomationControls aware of preset loading.