Variable view does not scale correctly with models with a lot of new RTD

The current Variable view is flat. It displays all Runtime data at the root of the view.
This doesn't scale correctly for languages with a lot of RTD or that create a lot of new RTD.

for example in the Logo (complete code in [dvojtise/mde-crashcourse-logo](https://github.com/dvojtise/mde-crashcourse-logo/tree/master/part3-mmfirst-solution))
The variable view grows by adding new segments to the drawing board. It is impossible to interpret the data displayed there especially after a high number of steps.

I suggest that the view should display only RTD which are not already contained (directly or indirectly) by another RTD. Thus the view should display only "root" RTD and use the nesting to display the rest.

Note1: the yellow highlighting about the last modified values should unfold the tree in order to focus on the appropriate data.
Note2: the same problem also occurs on the Multidimentionnal timeline which should probably take a similar nesting approach

Start by reproducing the scaling problem in the Variable view with the linked Logo example, then inspect the Variable view and Multidimensional timeline entry points. Done means displaying only root RTD values with nested descendants, unfolding the tree for yellow-highlighted changes, and applying a similar nesting approach to the timeline.

Written by the indexing model from the issue text.