Support for <graphic> element

[wergo]

At the moment (3.11.), Verovio does not read or write the <graphic> element within <surface>, although the other elements inside facsimile group (<facsimile>, <zone>) are supported. Thus, this element gets lost upon MEI input/output. See discussion here #2859.

It would be great to add support for an element to the Verovio codebase (creating graphic.cpp and graphic.h) and add support for input and output of it.