Remove ability to set multiple media items as song Linked Audio
This issue replaces issue #913
Currently users can set multiple audio files within the Song Edit Linked Audio tabs. However, when the song is displayed only the first audio file is actually played.
Issue #913 aimed to allow multiple audio files to be played, but when the functionality was developed (in merge request 364) it was considered that the change introduced too much complexity for what was a debatable requirement.
There doesn't seem to be a valid reason why a user would wish to associated multiple audio files with a song. It is conceivable that users would wish to associated multiple audio files with a series of images at the start or end of a service, but this isn't what would be provided by the song Linked Audio functionality. And that requirement could be met by using a music player application playing the audio while openlp cycles through the images.
Hence this issue is to change the edit song Linked Audio tab, so that only 1 audio file can be associated with the song. This will remove confusion for users.
If a song already has multiple linked audio files then only the first should be maintained when the data is saved.