Bug fix + improvements

[jeanineharb]

This pull request adds the following modifications:

• Implemented a fix for empty dropdown lists: not having initial data will not cause any more errors, however it is recommended to load the fixtures (either provided or custom) before serving the application.
• Linked states in the annotation bar to the corresponding timing.
• Added a delete button by annotation.
• Refactored the code to keep it modular.

[antingshen]

Great, thanks!