Add generic API to reindex fulltext

Currently to reindex the fulltext of a document one has to modify and save a document again (which reindexes the "simpletext"), and trigger the proper event (to reindex "binarytext"). In addition, listeners and other triggers should be deactivated while this is done (using a low-level Session is a way to do that).

This is done by the nuxeo-reindex-fulltext addon but this is cumbersome and specific to VCS.

There should be a generic API making it possible to write a simple addon reindexing the fulltext.

(Exposing this in the Admin Center would be another task.)