Improve first-run UX

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

imports/NeoChat/Component/Login/LoginStep.qml

@@ -0,0 +1,28 @@
+// SPDX-FileCopyrightText: 2020 Carl Schwan <carlschwan@kde.org>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+import QtQuick 2.14
+import QtQuick.Controls 2.14
+import QtQuick.Layouts 1.14
+
+/// Step for the login/registration flow
+ColumnLayout {
+
+    property string title: i18n("Welcome")
+    property string message: i18n("Welcome")
+    property bool showContinueButton: false
+    property bool showBackButton: false
+    property bool acceptable: false
+    property string previousUrl: ""
+
+    /// Process this module, this is called by the continue button.
+    /// Should call \sa processed when it finish successfully.
+    property Action action: null
+
+    /// Called when switching to the next step.
+    signal processed(url nextUrl)
+
+    signal showMessage(string message)
+
+}