GH-4071 Heavily refactor and rearchitect account system

This makes the account system much more modular
and makes it treat errors as something recoverable,
unless they come directly from the MSA refresh token
becoming invalid.

launcher/minecraft/auth/steps/YggdrasilStep.h

@@ -0,0 +1,28 @@
+#pragma once
+#include <QObject>
+
+#include "QObjectPtr.h"
+#include "minecraft/auth/AuthStep.h"
+
+class Yggdrasil;
+
+class YggdrasilStep : public AuthStep {
+    Q_OBJECT
+
+public:
+    explicit YggdrasilStep(AccountData *data, QString password);
+    virtual ~YggdrasilStep() noexcept;
+
+    void perform() override;
+    void rehydrate() override;
+
+    QString describe() override;
+
+private slots:
+    void onAuthSucceeded();
+    void onAuthFailed();
+
+private:
+    Yggdrasil *m_yggdrasil = nullptr;
+    QString m_password;
+};