The basic structure for event chain reductions has been implemented, as well
as functioning reductions for chained CodeChangeEvents.

For this, a visitor has been implemented which is fed a stream of events.
Every new event which is passed in is then chained with the previous event
if certain conditions apply.
For details, see: https://gitlab.informatik.uni-bremen.de/dfgpc/mimesis/-/wikis/events/Chain-Reductions

A setting has also been added allowing the user to choose whether or not
to apply reductions.

Additional Changes:
- When uploading a file, the file browser will now only display xml files.
- Better formatting of special characters in the event list's attributes.
- Lombok's @Getter and @Setter have been used to replace certain classes methods.