(Add option to) show node siblings in mode when filtering

[DPDmancul]

Checklist

• [X] I've looked through the documentation and existing open issues for this feature/feature request.

Describe the feature request

In the processes widget it would be useful to make search optionally work as "select" (as in vim, less, man, ...) instead of filtering. This would be very useful in tree view, where now it is very hard to both find a process and then see its children, siblings or cousins. In facts when searching it shows only matching processes and ancestors: to display children you have to exit from search, loosing the focus from the selected process.

[ClementTsang]

In regards to the tree comment, I already now show all parents/children on search on the master branch.

[DPDmancul]

This solved the 90% of cases, but few times it is useful to see also siblings processes. It's not a must have, but a nice to have feature.

[ClementTsang]

Hmm, maybe it might be easier to make a tree-specific search keyword to show siblings then? Just spitballing.