editor_services was growing into one interface every tool depended on
whole. It splits into the roles the editor plays for its tools: the
selection, the view, the undo history, the templates by the track, the
ground with the terrain model, the track design and the guide of the
first steps. Each tool is handed only the roles it uses, so what it
depends on can be read from its constructor; editor_services stays as
the sum of the roles the editor implements.
The first step of taking the tools out of editor_mode: a tool keeps its
own state and reaches the editor through editor_services, the few things
it needs from it, instead of through every member of the editor. The
editor holds its tools in a list for the hooks they share: a deleted
node is forgotten by all of them in one place, and a self-test command
the editor doesn't know goes to each of them.
The gauge moves with its state, its window and its self-test command;
the ImGui ids and the texts stay the same, and the self-test gives the
same hits on the test scenery as before.
