Indexer v3 Rewrite: Use the metadata index for backlinks; add INDEXER_METADATA_INDEX event

This new event allows plugins to add or modify the metadata that will be
indexed. Collecting this metadata in an event allows plugins to see if
other plugins have already added the metadata they need and leads to
just one single indexer call thus fewer files are read and written.
Plugins could also replace/prevent the metadata indexer call using this
event.