Make it possible to zoom backwards in time, excluding descendants from certain time/divergence onwards

[corneliusroemer]

Context
I wanted to analyse how SC2 evolved during the first few months, to see how pango lineages A and B are related, for example. However, the tree is very dense in that region and I couldn't find a way to zoom into the first few months.

It's possible to zoom into descendants of a certain node, but there seems to be no equivalent in the other direction: excluding descendants after a certain point.

One can filter with respect to time with beginning and end time points, but this doesn't change the view.

If I have missed a feature, please let me know. The problem may then just be discoverability of that feature or entirely with me.

Possible solution
The most straightforward way to introduce this feature would be to use the existing time filter and change view dynamically based on the cut-off times.

As a graphical example of what I mean here is a screenshot. I would like to zoom into the red box:

[trvrb]

Closing this issue in favor of #1368.