save-as: default to current session name.

this allows to easily add suffixes or otherwise tweak
the current session-name.

gtk2_ardour/ardour_ui.cc

@@ -2464,10 +2464,10 @@ ARDOUR_UI::save_session_as ()
 
 	if (!save_as_dialog) {
 		save_as_dialog = new SaveAsDialog;
-	} else {
-		save_as_dialog->clear_name ();
 	}
 
+	save_as_dialog->set_name (_session->name());
+
 	int response = save_as_dialog->run ();
 
 	save_as_dialog->hide ();

gtk2_ardour/save_as_dialog.cc

@@ -157,6 +157,13 @@ SaveAsDialog::clear_name ()
 	set_response_sensitive (RESPONSE_OK, false);
 }
 
+void
+SaveAsDialog::set_name (std::string name)
+{
+	new_name_entry.set_text (name);
+	name_entry_changed ();
+}
+
 bool
 SaveAsDialog::include_media () const
 {

gtk2_ardour/save_as_dialog.h

@@ -40,6 +40,7 @@ public:
 	bool copy_external () const;
 
 	void clear_name ();
+	void set_name (std::string);
 	
 private:
 	Gtk::CheckButton switch_to_button;