The fourth tool out of editor_mode. The track under the cursor and the
first steps of the tutorial become types of their own, which the editor
gives to the tools, and a tool shown as a tab of the lineside panel keeps
its open and expanded flags in a tool_tab. The trainsets waiting for the
save are handed to the editor by the tool, which hears back when they
were written. The self-test command trainset moves with it.
