Allow combined search on the wiki content of a document and the object data

Currently the lucene plugin creates 3 objets (document, object data, attachments) and indexes them separately. While it can seem ok to index the attachment data separately the indexing of the object data separately does not seem to bring benefits but has some functionnal issues:

1/ Duplicate search results (see XPLUCENE-5)
2/ Inability to combine a full text search or search on the wiki content with a search on a specific object field.

Example:

If I have a document with the content XYZT and tags TAGA, TAGB

I cannot do a search

XYZT AND XWiki.TagClass_tags:TAGA

since both data are indexes in separate objects.

An obvious solution seem to index the object data and the document data together. This design was done a VERY long time ago when the plugin was initially written by a contributor. I don't actually think there is a good reason for this separation.