Event Highlighting: Custom css will be overwritten after editor restart

[thgrund]

The added custom css within the styles.less file for the class event-highlighting, will be overwritten every time the TidalCycles package will be reloaded. This happens mainly, when the editor was restarted.

This happens, because we always set the default css properties during the launch of the event highlighting feature:
https://github.com/tidalcycles/pulsar-tidalcycles/blob/master/lib/event-highlighter.js#L143-L150

Because this happens only during startup, we could simply add a check and iterate over document.styleSheets to search for an already defined event-highlighting class within the global styleSheets.