_frontend: allow showing the element kind in bst show

src/buildstream/_frontend/cli.py

@@ -600,6 +600,7 @@ def show(app, elements, deps, except_, order, format_):
 
     \b
         %{name}           The element name
+        %{kind}           The element kind (Since: 2.6)
         %{description}    The element description, on a single line (Since: 2.3)
         %{key}            The abbreviated cache key (if all sources are consistent)
         %{full-key}       The full cache key (if all sources are consistent)

src/buildstream/_frontend/widget.py

@@ -355,6 +355,7 @@ def show_pipeline(self, dependencies, format_):
             description = " ".join(element._description.splitlines())
 
             line = p.fmt_subst(line, "name", element._get_full_name(), fg="blue", bold=True)
+            line = p.fmt_subst(line, "kind", element.get_kind(), fg="cyan")
             line = p.fmt_subst(line, "key", key.brief, fg="yellow", dim=dim_keys)
             line = p.fmt_subst(line, "full-key", key.full, fg="yellow", dim=dim_keys)
             line = p.fmt_subst(line, "description", description, fg="yellow", dim=dim_keys)

tests/frontend/show.py

@@ -42,6 +42,8 @@
         ("import-bin.bst", "%{name}", "import-bin.bst"),
         ("import-bin.bst", "%{state}", "buildable"),
         ("compose-all.bst", "%{state}", "waiting"),
+        ("import-bin.bst", "%{kind}", "import"),
+        ("compose-all.bst", "%{kind}", "compose"),
     ],
 )
 def test_show(cli, datafiles, target, fmt, expected):