expose the id attribute of credit in `mx::api`

[webern]

MusicXML 3.1 gives many elements an optional id attribute. mx::api exposes the id of elements that have a matching mx::api type (#399). <credit> was left out.

Today the value is dropped. Reading a file loses it, and there is no way to author one.

<credit> has an id, and so does each of its children <credit-words>, <credit-symbol> and <credit-image>. That is two id values on one credit, or more.

mx::api holds a credit as one object: ScoreData::pageTextItems holds PageTextData and ScoreData::pageImageItems holds PageImageData (src/include/mx/api/PageTextData.h, src/include/mx/api/PageImageData.h). Each one stands for a <credit> and its child together.

A <credit> may also hold more than one <credit-words> or <credit-symbol>. PageTextFunctions.cpp reads only the first one.