refactor(editor): the structure gauge is a tool of its own

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.

application/editortool.h

@@ -0,0 +1,52 @@
+/*
+This Source Code Form is subject to the
+terms of the Mozilla Public License, v.
+2.0. If a copy of the MPL was not
+distributed with this file, You can
+obtain one at
+http://mozilla.org/MPL/2.0/.
+*/
+
+#pragma once
+
+#include <cstddef>
+#include <sstream>
+#include <string>
+
+#include <glm/glm.hpp>
+
+class TTrack;
+namespace scene
+{
+class basic_node;
+}
+
+enum class editor_selftest_result
+{
+	unhandled,
+	handled,
+	pause
+};
+
+class editor_services
+{
+  public:
+	virtual TTrack *selected_track() const = 0;
+	virtual bool track_editing() const = 0;
+	virtual std::size_t history_size() const = 0;
+	virtual glm::dvec3 camera_position() const = 0;
+	virtual void fly_to(glm::dvec3 const &Target, glm::dvec3 const &Look, double const Distance, double const Duration = 0.6) = 0;
+
+  protected:
+	~editor_services() = default;
+};
+
+class editor_tool
+{
+  public:
+	virtual ~editor_tool() = default;
+	virtual void update() {}
+	virtual void render_windows() {}
+	virtual void forget(scene::basic_node const *Node) {}
+	virtual editor_selftest_result selftest(std::string const &Command, std::istringstream &Words) { return editor_selftest_result::unhandled; }
+};