Support annotations

[linuxcaffe]

Describe your improvement idea or feature request

a Taskwarrior task can have any number of annotations, and it's great to be able to associate notes on a task, beyond the description. This feature request is to extend that functionality to taskwarrior-flutter.

How would you like to see it implemented?

1. expose an annotation field in the task details view, with a button for New Annotation.
2. If the task has annotations, display each one at the bottom of the task details view.
3. on clicking New Annotation button, or existing annotation, open in a simple edit window (markdown supported?)
4. sync tasks with annotations, as usual.

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