Store audio file name so it can be used when exporting

[Kangaroux]

When the user opens an audio file we should store that file name on the project. Later when the project is exported we can save the audio file name in whatever format is being exported.

If the user imports a project and hasn't opened an audio file we should use the file name from the imported project. This is to make sure that if the user opens a project, edits it, and saves it without loading the audio file, we don't overwrite the audio file name.