Issue #1150260 by drunken monkey: Added a way to let processors and data alterations decide on which indexes they can run. (API CHANGE)

Follow-up to issue #1013018 by drunken monkey: Make the "Fulltext field" data alteration more useful.

Add $index->entityWrapper() helper method, for getting rid of all those long entity_metadata_wrapper() calls.

    * Distinction and interaction between service class and server object
          o Common server properties like id, name, machine name, etc., are now only stored in the server object.
          o The service object has now a defined constructor, which contains the corresponding server object as a parameter.
          o The abstract service class saves $server and a reference to $server->options (as $options) in its constructor.
          o When a service class wants to access other server properties (apart from its own settins), e.g. the server ID, it should just use $this->server->id.
    * Service configuration form handling
          o The signatures of the three SearchApiServiceInterface::configurationForm*() methods have changed.
          o $prefix is gone, simply use form_error() instead.
          o SearchApiServiceInterface::configurationFormSubmit() has a new purpose, as it is now responsible for storing the new service options itself. This is most easily done (when extending the abstract service class) by setting $this->options appropriately.
          o If you haven't overridden these methods, you can ignore these changes – the abstract service's default implementation works just as before.
    * Renamed SearchApiServiceInterface::view() to SearchApiServiceInterface::viewSettings().