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/flows/MSA.h

@@ -0,0 +1,22 @@
+#pragma once
+#include "AuthFlow.h"
+
+class MSAInteractive : public AuthFlow
+{
+    Q_OBJECT
+public:
+    explicit MSAInteractive(
+        AccountData *data,
+        QObject *parent = 0
+    );
+};
+
+class MSASilent : public AuthFlow
+{
+    Q_OBJECT
+public:
+    explicit MSASilent(
+        AccountData * data,
+        QObject *parent = 0
+    );
+};