Add threaded session backend architecture with real SSH backend

src/unsupported_session_backend.cpp

@@ -0,0 +1,26 @@
+#include "unsupported_session_backend.h"
+
+UnsupportedSessionBackend::UnsupportedSessionBackend(const Profile& profile, QObject* parent)
+    : SessionBackend(profile, parent)
+{
+}
+
+void UnsupportedSessionBackend::connectSession(const SessionConnectOptions&)
+{
+    const QString message = QStringLiteral("Protocol '%1' is not implemented yet.")
+                                .arg(profile().protocol);
+    emit eventLogged(message);
+    emit stateChanged(SessionState::Failed, message);
+    emit connectionError(message, message);
+}
+
+void UnsupportedSessionBackend::disconnectSession()
+{
+    emit stateChanged(SessionState::Disconnected,
+                      QStringLiteral("No active connection for this protocol."));
+}
+
+void UnsupportedSessionBackend::reconnectSession(const SessionConnectOptions& options)
+{
+    connectSession(options);
+}