Add read only connections in Wiring Editor

Signed-off-by: Álvaro Arranz <aarranz@conwet.com>
Add read only tests for Wiring Editor

Signed-off-by: Álvaro Arranz <aarranz@conwet.com>
Fix typo in test_wiring_show_invisible_operator_with_connections

Signed-off-by: Álvaro Arranz <aarranz@conwet.com>