_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.

Fixes #890 (closed).