add ::time_domain() methods to Playlist and Region

These are heuristics based on data type for now. That may evolve over time,
but it's a reasonable place to begin

libs/ardour/ardour/playlist.h

@@ -112,6 +112,8 @@ public:
 	bool set_name (const std::string& str);
 	void set_region_ownership ();
 
+	Temporal::TimeDomain time_domain() const;
+
 	/*playlist group IDs (pgroup_id) is a group identifier that is implicitly
 	 * or explicitly assigned to playlists so they can be associated with each other.
 	 *

libs/ardour/ardour/region.h

@@ -108,6 +108,7 @@ public:
 	bool set_name (const std::string& str);
 
 	const DataType& data_type () const { return _type; }
+	Temporal::TimeDomain time_domain() const;
 
 	/** How the region parameters play together:
 	 *

libs/ardour/playlist.cc

@@ -3446,3 +3446,16 @@ Playlist::rdiff_and_add_command (Session* session)
 	session->add_commands (cmds);
 	session->add_command (new StatefulDiffCommand (shared_from_this ()));
 }
+
+Temporal::TimeDomain
+Playlist::time_domain() const
+{
+	switch (_type) {
+	case DataType::AUDIO:
+		return Temporal::AudioTime;
+	default:
+		break;
+	}
+
+	return Temporal::BeatTime;
+}

libs/ardour/region.cc

@@ -2068,3 +2068,22 @@ Region::region_relative_position (timepos_t const & p) const
 	*/
 	return p.earlier (position());
 }
+
+Temporal::TimeDomain
+Region::time_domain() const
+{
+	boost::shared_ptr<Playlist> pl (_playlist.lock());
+
+	if (pl) {
+		return pl->time_domain ();
+	}
+
+	switch (_type) {
+	case DataType::AUDIO:
+		return Temporal::AudioTime;
+	default:
+		break;
+	}
+
+	return Temporal::BeatTime;
+}