ES02: extend Track and Artist with copy constructors, implement Event class

# Please enter the commit message for your changes. Lines starting
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# Date:      Sun May 1 01:52:34 2022 +0200
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# On branch master
# Changes to be committed:
#	modified:   src/MusicLandscape/entities/Artist.java
#	new file:   src/MusicLandscape/entities/Event.java
#	modified:   src/MusicLandscape/entities/Track.java
#
