Plugin tree depth, new context propperty

[JoranAngevaare]

For several applications it would be nice to have an index of "how deep" a plugin is.

For example for the datastructure in the straxen documentation we need such a depth to build the plugin tree: https://straxen.readthedocs.io/en/latest/reference/datastructure_nT.html. Another example is bootstrax, where we would like to sort the plugins to have the highest level targets first.

One complication might be that a plugin tree might not might not (in fact amost never) has one single final plugin. As such the "tree" might be ambiguous and a forest might be the better description.