Support querying element children in Extended search

[MartinHammarstedt]

Given the following corpus data, we want to be able to find all tokens that are children of <person> elements:

Han heter <person>Kalle</person>.

Currently, we have to rely on an awkward workaround: modifying the source data to include a dummy attribute:

Han heter <person dummy="-">Kalle</person>.

…together with the following Korp config:

- person_dummy:
    dataset:
      - '-'
    extended_component: singleValue

It would be better to have a solution that does not require modifying the corpus data.

In CQP, searching for children of an element is simple:

[person]

And to find tokens that are not children of an element:

[!person]

The main question is how this option should be presented to users in Extended search.