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/qml/Dialog/ConfirmLogout.qml

@@ -37,7 +37,7 @@ QQC2.Dialog {
             text: i18n("Sign out")
             QQC2.DialogButtonBox.buttonRole: QQC2.DialogButtonBox.AcceptRole
             onClicked: {
-                Controller.logout(root.connection, true);
+                root.connection.logout(true);
                 root.close();
                 root.accepted();
             }