Display gain and midiNote plugin parameters/properties nicely.

Show fancy values on generic GUI controls, automation lane controls, and automation lane verbose cursor.
Fix text display of midiNote values.
Make bigstep of midiNote parameters 12 (one octave).
Add ARDOUR::value_as_string() as a stateless one-stop-shop for value printing.

gtk2_ardour/automation_line.h

@@ -34,6 +34,7 @@
 #include "pbd/memento_command.h"
 
 #include "ardour/automation_list.h"
+#include "ardour/parameter_descriptor.h"
 #include "ardour/types.h"
 
 #include "canvas/types.h"
@@ -64,6 +65,7 @@ public:
 	                TimeAxisView&                                      tv,
 	                ArdourCanvas::Item&                                parent,
 	                boost::shared_ptr<ARDOUR::AutomationList>          al,
+	                const ARDOUR::ParameterDescriptor&                 desc,
 	                Evoral::TimeConverter<double, ARDOUR::framepos_t>* converter = 0);
 
 	virtual ~AutomationLine ();
@@ -234,6 +236,8 @@ private:
 	/** maximum time that a point on this line can be at, relative to the position of its region or start of its track */
 	ARDOUR::framecnt_t _maximum_time;
 
+	const ARDOUR::ParameterDescriptor _desc;
+
 	friend class AudioRegionGainLine;
 };