simplay-web: organize event list to timestamp dictionary

[moschi]

The current implementation of the loop which works through the events of the current timestamp is not optimized for performance. Within each step, the events for the current timestamp are collected by filtering the array. This causes some performance issues in large collections of events. This task aims to pre-sort the events for each occurring timestamp once, at the beginning of the visualization, so that the events for a timestamp are easily accessible.

DoD:

• events are only sorted for timestamp once