Add output-attributes grob property to replace id

input/regression/output-attributes.ly

Index: input/regression/output-attributes.ly
diff --git a/input/regression/output-attributes.ly b/input/regression/output-attributes.ly
new file mode 100644
index 0000000000000000000000000000000000000000..40682940662119c92456746aa695716b28296fd0
--- /dev/null
+++ b/input/regression/output-attributes.ly
@@ -0,0 +1,15 @@
+\version "2.17.6"
+
+\header {
+  texidoc = "Shows the output-attributes property of a grob being set.
+This should have no effect in the Postscript backend.  In the SVG
+backend these settings should produce this group tag:
+@code{<g id=\"123\" class=\"foo\" data-whatever=\"bar\"> @dots{} </g>}
+"
+}
+
+{
+  \override NoteHead.output-attributes =
+  #'((id . 123) (class . foo) (data-whatever . "bar"))
+  c
+}