_frontend: Allow printing dependencies using `bst show`

At present, there isn't an easy way to print anything from `bst show`
that would give the users an idea of what the dependency graph looks
like. One could use things like `--deps build`, but that will just print
a list, without any information about the dependency edges.

Add `%{deps}`, `%{build-deps}` and `%{runtime-deps}` format strings to
`bst show` that would simply print the list of all dependencies, build
dependencies and runtime dependencies respectively.

Summary of changes:

  * buildstream/_frontend/cli.py: Add help for new format symbols.
  * buildstream/_frontend/widget.py: Add support for new format symbols
                                     for dependencies.
  * tests/frontend/show.py: Add tests for new format symbols.

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