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.

_frontend: Allow printing dependencies using `bst show`

Closes #890

See merge request !1121

Fedora 29 includes python 3.7 so this is not needed anymore

.gitlab-ci.yml: Test with current fedora release: 29

See merge request !1137

This will allow cross-junction dependencies to be listed as strings on a
single line.

* _loader/types.py: While initializing `Dependency` objects, attempt to
                    split filenames, only if no `junction` was specified
                    explicitly. If a `junction` was specified, then
                    filenames with `:` in their names will result in an
                    error.

* tests/frontend/buildcheckout.py: Add tests to ensure the above.

* _versions.py: Bump BST_FORMAT_VERSION.

Fixes #809.

In the previous commit, we added support to express cross-junction
dependencies inline as simple strings. Document it along with the
version in which the feature was added.