Introduce NeoChatConnection

Previously, some functions that conceptually belong to the connection needed to be in the Controller, since we didn't have a place to put them.
This fixes that by extending the Connection class in a similar way as we extend the Room class.

src/registration.cpp

@@ -89,7 +89,7 @@ void Registration::registerAccount()
     connect(job, &BaseJob::result, this, [=]() {
         if (job->status() == BaseJob::Success) {
             setNextStep("loading"_ls);
-            auto connection = new Connection(this);
+            auto connection = new NeoChatConnection(this);
             auto matrixId = "@%1:%2"_ls.arg(m_username, m_homeserver);
             connection->resolveServer(matrixId);