Create annotation that targets the entire score

[alastair]

Initially reported in #9

The <AnnotationSubmitter> expects to receive a list of targets (i.e. selected notes or measures) that are being annotated. If no items are being selected, the annotation should apply to the whole file.
We should ensure that there is some indication about what is happening in this case - maybe some text that pops up if you start writing an annotation - ("This annotation will apply to the whole score"), or a specific checkbox that the user has to select to indicate that they want to annotate everything.

If some types of annotation can only be applied to a whole score, or only to a specific element (or elements), then we should disable them depending on what music elements are currently selected.