extend step sequencer API to start data mods

libs/ardour/ardour/step_sequencer.h

@@ -85,6 +85,8 @@ class Step {
 	void set_timeline_offset (Temporal::Beats const &, Temporal::Beats const &);
 
   private:
+	friend class StepSequence; /* HACK */
+
 	StepSequence&      _sequence;
 	bool               _enabled;
 	Temporal::Beats     timeline_offset;
@@ -142,6 +144,8 @@ class StepSequence
 
 	void startup (Temporal::Beats const & start, Temporal::Beats const & offset);
 
+	void adjust_step_pitch (int step, int amt);
+
 	Temporal::Beats bar_size() const { return _bar_size; }
 
 	double root() const { return _root; }
@@ -212,6 +216,9 @@ class StepSequencer {
 
 	TempoMap& tempo_map() const { return _tempo_map; }
 
+	/* editing */
+	void adjust_step_pitch (int seq, int step, int amt);
+
   private:
 	Glib::Threads::Mutex       _sequence_lock;