Improve first-run UX

- Replace LoginPage with step-by-step approach to support different login flows
- Implement login using SSO

imports/NeoChat/Component/Login/Loading.qml

@@ -0,0 +1,23 @@
+/**
+ * SPDX-FileCopyrightText: 2020 Tobias Fella <fella@posteo.de>
+ *
+ * SPDX-License-Identifier: GPL-3.0-only
+ */
+import QtQuick 2.15
+import QtQuick.Controls 2.12 as QQC2
+import QtQuick.Layouts 1.12
+
+import org.kde.neochat 1.0
+
+import NeoChat.Component 1.0
+
+import org.kde.kirigami 2.12 as Kirigami
+
+QQC2.BusyIndicator {
+
+    property var showContinueButton: false
+    property var showBackButton: false
+    property string title: i18n("Loading")
+
+    anchors.centerIn: parent
+}