Add widget and operator add/remove/fail events in WiringEditor

Signed-off-by: Álvaro Arranz <aarranz@conwet.com>
Allow to pass a second position to UserAction
Improve Basic Concept Tutorial using the new WiringEditor events

Signed-off-by: Álvaro Arranz <aarranz@conwet.com>
Fix bug in the Basic concepts tutorial raised when not using the oiltheme
Fix regression in Arrow