Improve initial active connection handling

If the last active connection is not reachable (server down, keychain problems,
token revoked, etc.), NeoChat currently fails to load at all, with the only fix
being to delete a line from the config file. This is surprisingly hard to fix with
a nice UX as long as we stick to the principle of loading the user's last active
connection automatically.

This patch thus drops that principle; instead, the user is always asked to choose
the connection to continue with.

src/login.h

@@ -79,7 +79,17 @@ class LoginHelper : public QObject
     Q_PROPERTY(bool isInvalidPassword READ isInvalidPassword NOTIFY isInvalidPasswordChanged)
 
 public:
-    explicit LoginHelper(QObject *parent = nullptr);
+    static LoginHelper &instance()
+    {
+        static LoginHelper _instance;
+        return _instance;
+    }
+
+    static LoginHelper *create(QQmlEngine *engine, QJSEngine *)
+    {
+        engine->setObjectOwnership(&instance(), QQmlEngine::CppOwnership);
+        return &instance();
+    }
 
     Q_INVOKABLE void init();
 
@@ -125,6 +135,7 @@ Q_SIGNALS:
     void isLoggingInChanged();
     void isLoggedInChanged();
     void isInvalidPasswordChanged();
+    void loaded();
 
 private:
     void setHomeserverReachable(bool reachable);
@@ -141,4 +152,5 @@ private:
     bool m_isLoggingIn = false;
     bool m_isLoggedIn = false;
     bool m_invalidPassword = false;
+    explicit LoginHelper(QObject *parent = nullptr);
 };