Modify indexer for documents so that it queries for our own IDocumentIndexer adapter.

This allows us to choose whether to index original documents or their preview PDF, depdending on whether `opengever.pdfconverter` is installed or not.

- Default Behavior in `opengever.core`: `DefaultDocumentIndexer` from `opengever.document` is used. This indexer extracts the plain text from the original documents, using a portal transform to `text/plain`.
- Behavior if `opengever.pdfconverter` is installed: Because the documents then provide the `IPreviewMarker` interface, the `OfficeDocumentFromPDFIndexer` from `opengever.pdfconverter` will be selected. This indexer extracts the text from the preview PDF of the adapted document.

TODO
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- [ ] Prevent indexer from being executed multiple times.