A .ai file that File > Export As, its dialog or a control request writes
to a path now names the view's active artboard, as document.export and
document.serialize do. The UI export records the view's artboard and
takes its document from fileio::export_doc, which applies
DocState::active_artboard to the export's copy; document.export and
document.serialize use the same function.
The app now records the view's active artboard before every command,
including the commands the UI handles itself, so a .ai copy from Save a
Timestamped Copy or Export All Open Documents names it too. Before,
these files named the artboard the document opened on, or the first.
The open document is unchanged.