The Annotations panel is the most obvious place to perform actions
related to annotations, so let's make that possible. :^)
The toolbar gets open/save/save-as actions for annotations, and one for
adding an annotation. The table itself gets a context menu for editing
or deleting the selected annotation.
This is a fairly simple JSON format: A single array, containing objects,
with the Annotation fields as key:value pairs.
When reading a file, we let invalid or missing keys fall back to the
default values. This is mostly intended to set a pattern so that if we
add new fields in the future, we won't fail to load old annotations
files. If loading the file fails though, we keep the previously loaded
set of annotations.
