Support configuring the SearchIndexListener behaviour

[Toflar]

This will allow to configure how the SearchIndexListener behaves.
Right now it updates the index when the response was successful and it deletes it from the index, if it was not.

If you have cronjobs doing that work for you, you may want to disable that listener completely or have it e.g. only delete from the index. You can configure all 4 possible states using the config now:

contao:
    search:
        listener:
            index: true
            delete: false

[leofeyer]

I think I would prefer to configure the listener like this:

contao:
    search:
        default_indexer:
            enable: true
        listener:
            index: true
            delete: true

IMHO this is more understandable and less error-prone than a string.

[Toflar]

Configuration simplified as suggested in cf3de3b.

[Toflar]

All tests pass, merged master too. This should be RTM.

[leofeyer]

Thank you @Toflar.