Add smufl attribute to more percussion child elements

[mdgood]

Although some child elements of the <percussion> element have a smufl attribute, such as <glass>, others do not. In particular, the <effect>, <membrane>, <metal>, <timpani>, and <wood> elements do not have smufl attributes, even though there are SMuFL stylistic alternates for at least one of their element values.

This means that we cannot capture the SMuFL stylistic alternate being used without resorting to the <other-percussion> element and thus losing semantics that could be captured.

Adding the smufl attribute would let us keep the full encoding of the semantics while also encoding the exact SMuFL glyph being used.