Restructure the search application's layout

The current xwiki-platform-search-ui contains lucene search UI documents that should be in a dedicated search-lucene-ui module. Also, as we did for solr by grouping the solr related modules under the xwiki-platform-search-solr parent module, so should we do with the lucene module.

The newly proposed layout is the following:

xwiki-platform-search
- xwiki-platform-search-solr
-- xwiki-platform-search-solr-ui
-- xwiki-platform-search-solr-api
-- xwiki-platform-search-solr-query
- xwiki-platform-search-lucene
-- xwiki-platform-search-lucene-api  (renamed from xwiki-platform-search-lucene)
-- xwiki-platform-search-lucene-ui   (contains pages extracted from xwiki-platform-search-ui)
- xwiki-platform-search-ui

xwiki-platform-search-ui should generally contain implementation-agnostic pages, but also the Main.DatabaseSearch implementation that should be considered the default search UI implementation.