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.
