lib: fix stupid bug in `Persistent_state.archive`
lib: add submodule `Target.Is` (boolean queries)
lib: add function `State.archived_targets`
lib: start implementing the `interact` subcommand
lib: add function `Target.Is.killable`
lib: add function `Target.name`
lib: add function `State.get_target`

There is an internal function which was called `get_target` and has been
renamed to `get_target_with_db`.
lib: add `kill` action to the “target explorer”
lib: add function `State.is_archived`
lib: add `archive` to the Target Explorer
lib: fix cancelling in target-explorer