changes to global content in lively is slowed down, probably due to mutation observer

the rectangles benchmark showed that something is slowing down modifications of html elements in lively compared to running the same benchmark outside.

When running the benchmarks inside a shadow root, the performance gets much better, so we suspect the mutation observer to take its toll.

Start with the rectangles benchmark and compare modifications of global HTML content in lively with the same benchmark outside lively and inside a shadow root. Trace the mutation observer activity involved; done means the global-content benchmark no longer shows the reported slowdown or the cause is documented with measurements.

Written by the indexing model from the issue text.