add libardour infrastructure for "fade range" edit operation

libs/ardour/ardour/audioregion.h

@@ -115,6 +115,8 @@ class LIBARDOUR_API AudioRegion : public Region
 	XMLNode& get_basic_state ();
 	int set_state (const XMLNode&, int version);
 
+	void fade_range (framepos_t, framepos_t);
+
 	bool fade_in_is_default () const;
 	bool fade_out_is_default () const;
 

libs/ardour/ardour/playlist.h

@@ -142,6 +142,7 @@ public:
 	void nudge_after (framepos_t start, framecnt_t distance, bool forwards);
 	boost::shared_ptr<Region> combine (const RegionList&);
 	void uncombine (boost::shared_ptr<Region>);
+	void fade_range (std::list<AudioRange>&);
 
 	void shuffle (boost::shared_ptr<Region>, int dir);
 	void ripple (framepos_t at, framecnt_t distance, RegionList *exclude);

libs/ardour/ardour/region.h

@@ -224,6 +224,8 @@ class LIBARDOUR_API Region
 	void trim_end (framepos_t new_position);
 	void trim_to (framepos_t position, framecnt_t length);
 
+	virtual void fade_range (framepos_t, framepos_t) {}
+
 	void cut_front (framepos_t new_position);
 	void cut_end (framepos_t new_position);
 

libs/ardour/audioregion.cc

@@ -957,6 +957,34 @@ AudioRegion::set_state (const XMLNode& node, int version)
 	return _set_state (node, version, what_changed, true);
 }
 
+void
+AudioRegion::fade_range (framepos_t start, framepos_t end)
+{
+	framepos_t s, e;
+
+	switch (coverage (start, end)) {
+	case Evoral::OverlapStart:
+		s = _position;
+		e = end;
+		set_fade_in (FadeConstantPower, e - s);
+		break;
+	case Evoral::OverlapEnd:
+		s = start;
+		e = _position + _length;
+		set_fade_out (FadeConstantPower, e - s);
+		break;
+	case Evoral::OverlapInternal:
+		/* needs addressing, perhaps. Difficult to do if we can't
+		 * control one edge of the fade relative to the relevant edge
+		 * of the region, which we cannot - fades are currently assumed
+		 * to start/end at the start/end of the region
+		 */
+		break;
+	default:
+		return;
+	}
+}
+
 void
 AudioRegion::set_fade_in_shape (FadeShape shape)
 {

libs/ardour/playlist.cc

@@ -3123,6 +3123,16 @@ Playlist::uncombine (boost::shared_ptr<Region> target)
 	thaw ();
 }
 
+void
+Playlist::fade_range (list<AudioRange>& ranges)
+{
+	 for (list<AudioRange>::iterator r = ranges.begin(); r != ranges.end(); ++r) {
+		 for (RegionList::const_iterator i = regions.begin(); i != regions.end(); ++i) {
+			 (*i)->fade_range ((*r).start, (*r).end);
+		 }
+	 }
+}
+
 uint32_t
 Playlist::max_source_level () const
 {