Prepare to eventually drop Automatable::value_as_string()

Probably due to historical reasons, there are two APIs to format a
control's value. In all both variants end up calling either
ARDOUR::value_as_string() or the Controllable's formatting function
(except for panners).

libs/ardour/ardour/automation_control.h

@@ -116,6 +116,8 @@ class LIBARDOUR_API AutomationControl
 	double internal_to_interface (double i) const;
 	double interface_to_internal (double i) const;
 
+	virtual std::string get_user_string() const;
+
 	const ParameterDescriptor& desc() const { return _desc; }
 
 	const ARDOUR::Session& session() const { return _session; }

libs/ardour/automation_control.cc

@@ -31,6 +31,7 @@
 #include "ardour/event_type_map.h"
 #include "ardour/session.h"
 #include "ardour/selection.h"
+#include "ardour/value_as_string.h"
 
 #include "pbd/i18n.h"
 
@@ -338,6 +339,12 @@ AutomationControl::interface_to_internal (double val) const
 	return val;
 }
 
+std::string
+AutomationControl::get_user_string () const
+{
+	return ARDOUR::value_as_string (_desc, get_value());
+}
+
 void
 AutomationControl::set_group (boost::shared_ptr<ControlGroup> cg)
 {

libs/ardour/pannable.cc

@@ -28,6 +28,7 @@
 #include "ardour/panner.h"
 #include "ardour/pan_controllable.h"
 #include "ardour/session.h"
+#include "ardour/value_as_string.h"
 
 #include "pbd/i18n.h"
 
@@ -282,5 +283,5 @@ Pannable::value_as_string (boost::shared_ptr<const AutomationControl> ac) const
 		return p->value_as_string (ac);
 	}
 
-	return Automatable::value_as_string (ac);
+	return ARDOUR::value_as_string(ac->desc(), ac->get_value());
 }