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/GetSkinStep.cpp

@@ -0,0 +1,43 @@
+
+#include "GetSkinStep.h"
+
+#include <QNetworkRequest>
+
+#include "minecraft/auth/AuthRequest.h"
+#include "minecraft/auth/Parsers.h"
+
+GetSkinStep::GetSkinStep(AccountData* data) : AuthStep(data) {
+
+}
+
+GetSkinStep::~GetSkinStep() noexcept = default;
+
+QString GetSkinStep::describe() {
+    return tr("Getting skin.");
+}
+
+void GetSkinStep::perform() {
+    auto url = QUrl(m_data->minecraftProfile.skin.url);
+    QNetworkRequest request = QNetworkRequest(url);
+    AuthRequest *requestor = new AuthRequest(this);
+    connect(requestor, &AuthRequest::finished, this, &GetSkinStep::onRequestDone);
+    requestor->get(request);
+}
+
+void GetSkinStep::rehydrate() {
+    // NOOP, for now.
+}
+
+void GetSkinStep::onRequestDone(
+    QNetworkReply::NetworkError error,
+    QByteArray data,
+    QList<QNetworkReply::RawHeaderPair> headers
+) {
+    auto requestor = qobject_cast<AuthRequest *>(QObject::sender());
+    requestor->deleteLater();
+
+    if (error == QNetworkReply::NoError) {
+        m_data->minecraftProfile.skin.data = data;
+    }
+    emit finished(AccountTaskState::STATE_SUCCEEDED, tr("Got skin"));
+}