DERSL: Set the update timestamp in the constructor

This means that changing the size of the displayed slides through
renderSlides won't count as an actual modification of the data.
(Changing the dimensions doesn't equate to changing the underlying
EventResults.)
DERSL: Pass constructor parameter evr through a copy constructor

We don't want to maintain references to the EventResults. (ColorScheme
is unmodifiable, so it's not as important there.)