Honouring manually modified labels. Closes #1.

Previously, adding a track would cause a new "default" label to be generated for the project, even after a label had been manually entered by the user.

With this patch, user-inputted labels are no longer overwritten by "default" labels. Looking at the code, this seems to be the intended behaviour. I haven't fully tested how it behaves when saving and loading projects to and from the file system.