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/AuthStep.h

@@ -0,0 +1,33 @@
+#pragma once
+#include <QObject>
+#include <QList>
+#include <QNetworkReply>
+
+#include "QObjectPtr.h"
+#include "minecraft/auth/AccountData.h"
+#include "AccountTask.h"
+
+class AuthStep : public QObject {
+    Q_OBJECT
+
+public:
+    using Ptr = shared_qobject_ptr<AuthStep>;
+
+public:
+    explicit AuthStep(AccountData *data);
+    virtual ~AuthStep() noexcept;
+
+    virtual QString describe() = 0;
+
+public slots:
+    virtual void perform() = 0;
+    virtual void rehydrate() = 0;
+
+signals:
+    void finished(AccountTaskState resultingState, QString message);
+    void showVerificationUriAndCode(const QUrl &uri, const QString &code, int expiresIn);
+    void hideVerificationUriAndCode();
+
+protected:
+    AccountData *m_data;
+};