`getClef` & `getEndClef` in `Stave` do not provide all the information required to build a `Clef`

[rvilarl]

Both setClef and setEndClef have the following parameters:

clefSpec: string, size?: string, annotation?: string

getClef and getEndClef only return the clefSpec

[rvilarl]

Also Clef provides no method to get the parameters above used in its constructor.

[rvilarl]

I believe that it should be allowed to get the three parameters from getClef, getEndClef and Clef.

[rvilarl]

This also relates somehow to #580 I believe we are missing a consitent way to copy elements. I have think about the following possibilities:

• use overlay copy contructors in all classes like in Renderer. This would allow to make a copy of Clef. getClef and getEndClef could return the Clef object and then make a copy of it.
• have a static copy function in the elements with the same purpose.

Thoughts? Do you have other ideas?