Re-implement events

The current code is awkward, because it has been conceived around the idea of a “group” of annotations, which was meant to be an entity of code with special features, like having a list of annotations associated to it. This concept has evolved to become an “event”. Each event must have a unique label, which appears as the label of the annotation rectangle on the timeline. Each event in a given timeline should have a specific color, in order to improve the visualization experience, but this is not mandatory.

In the current implementation, each timeline contains a list of events, stored as a field of the Timeline class. It will be more logical to store the events as a dictionary, where the label is the dictionary key. The dictionary can be abstracted by the creation of a new class named, for instance, EventCollection. Each element of this dictionary would also be a dictionary, with fields color and description. This later would help users to know what an event is intended to code (for instance, the event with label 1 would have a description like “look on the left side”).

N.B.: a change like that will imply a bump in the FORMAT version.

Finally, the code in the classes EventDialog, ChooseEvent and ChangeEvent must be unified.

Notice that, in the case where this issue is addressed, then issue #100 (closed) may become obsolete.