Parent and child tags

Similar to how filesystem can have parent and child folders, tags should be capable of linking as well.

Each tag will be able to have multiple parents and multiple children. This change makes it possible to construct graphs of tags, where labeling a resource with, e.g. "Pushkin", can automatically label the resource with "Literature" and/or "Russian" tags. This dependency graph is defined only by user, so it is up to them to create graph which makes sense to them.

This dependency graph notion also affects `[tag selector]` which must also select all children tags of any selected tag. E.g. if a user selects "Literature", they are also presented by resources labeled as "Pushkin", "Shakespeare", "Remarque" etc. But not in _focus mode_ (see ARK-Builders/ark-components-android#17).

Graph structure of tags must be persisted and replicated, it can be stored in e.g. `.ark/graph`.

Start by examining the existing [tag selector] behavior and the focus-mode distinction described in ARK-Builders/ark-components-android#17. Define how the user-owned parent/child graph is persisted in `.ark/graph`, replicated, and used so selectors include descendants except in focus mode; completion requires all of those behaviors to work together.

Written by the indexing model from the issue text.