feat(editor): Help > Tutorial instead of First steps: seven lessons (looking around, laying track, curves, joining, switches, selecting and fixing, saving and driving), each with a looping clip from lang/editor_tutorial/<lesson>.gif played from the start on a click, the mouse buttons and keys drawn as pictures beside what they do, ticks that fill in when the user does it, a button that turns the right tool on, Back / Next and a dot per lesson that turns green when it is done

editor/editorClips.hpp

@@ -0,0 +1,56 @@
+/*
+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 <atomic>
+#include <cstdint>
+#include <memory>
+#include <string>
+#include <vector>
+
+namespace editor_clips
+{
+
+class clip
+{
+  public:
+	clip() = default;
+	~clip();
+	clip(clip const &) = delete;
+	clip &operator=(clip const &) = delete;
+
+	void open(std::string const &Path);
+	void close();
+	std::uint64_t texture(double const Seconds);
+	bool loading() const;
+	bool missing() const;
+	int width() const;
+	int height() const;
+	std::string const &path() const
+	{
+		return m_path;
+	}
+
+  private:
+	struct frames
+	{
+		int width{0};
+		int height{0};
+		std::vector<std::uint8_t> rgba;
+		std::vector<int> delays;
+		std::atomic<bool> done{false};
+	};
+	std::string m_path;
+	std::shared_ptr<frames> m_frames;
+	std::uint64_t m_texture{0};
+	int m_shown{-1};
+};
+
+} // namespace editor_clips