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/MSAStep.h

@@ -0,0 +1,32 @@
+
+#pragma once
+#include <QObject>
+
+#include "QObjectPtr.h"
+#include "minecraft/auth/AuthStep.h"
+
+#include <katabasis/DeviceFlow.h>
+
+class MSAStep : public AuthStep {
+    Q_OBJECT
+public:
+    enum Action {
+        Refresh,
+        Login
+    };
+public:
+    explicit MSAStep(AccountData *data, Action action);
+    virtual ~MSAStep() noexcept;
+
+    void perform() override;
+    void rehydrate() override;
+
+    QString describe() override;
+
+private slots:
+    void onOAuthActivityChanged(Katabasis::Activity activity);
+
+private:
+    Katabasis::DeviceFlow *m_oauth2 = nullptr;
+    Action m_action;
+};