GH-3392 dirty initial MSA support that shares logic with Mojang flows

Both act as the first step of AuthContext.

launcher/minecraft/auth/flows/MSAHelper.txt

@@ -0,0 +1,51 @@
+class Helper : public QObject {
+    Q_OBJECT
+
+public:
+    Helper(MSAFlows * context) : QObject(), context_(context), msg_(QString()) {
+        QFile tokenCache("usercache.dat");
+        if(tokenCache.open(QIODevice::ReadOnly)) {
+            context_->resumeFromState(tokenCache.readAll());
+        }
+    }
+
+public slots:
+    void run() {
+        connect(context_, &MSAFlows::activityChanged, this, &Helper::onActivityChanged);
+        context_->silentSignIn();
+    }
+
+    void onFailed() {
+        qDebug() << "Login failed";
+    }
+
+    void onActivityChanged(Katabasis::Activity activity) {
+        if(activity == Katabasis::Activity::Idle) {
+            switch(context_->validity()) {
+                case Katabasis::Validity::None: {
+                    // account is gone, remove it.
+                    QFile::remove("usercache.dat");
+                }
+                break;
+                case Katabasis::Validity::Assumed: {
+                    // this is basically a soft-failed refresh. do nothing.
+                }
+                break;
+                case Katabasis::Validity::Certain: {
+                    // stuff got refreshed / signed in. Save.
+                    auto data = context_->saveState();
+                    QSaveFile tokenCache("usercache.dat");
+                    if(tokenCache.open(QIODevice::WriteOnly)) {
+                        tokenCache.write(context_->saveState());
+                        tokenCache.commit();
+                    }
+                }
+                break;
+            }
+        }
+    }
+
+private:
+    MSAFlows *context_;
+    QString msg_;
+};