Add support for ghost widgets

Signed-off-by: Álvaro Arranz <aarranz@conwet.com>
Add test_wiring_show_invisible_widget and test_wiring_show_invisible_widget_with_connections

Signed-off-by: Álvaro Arranz <aarranz@conwet.com>
Fix pep8 problems
Improve ghosts visualization

Signed-off-by: Álvaro Arranz <aarranz@conwet.com>
Add test_wiring_show_invisible_operator and test_wiring_show_invisible_operator_with_connections

Signed-off-by: Álvaro Arranz <aarranz@conwet.com>
Fix some pep8 problems