Also adjust Sphinx keywords splitting to be consistent with other search
backends when it comes to handling hyphen (like ignoring hyphen when it hasn't
NOT meaning and ignoring hyphen wrapped with "plus" signs)
PHPBB3-16941
[ticket/8610] Merging topics does not handle bookmarks correctly
* Nicofuma/ticket/8610:
[ticket/8610] Use css selector instead of xPath
[ticket/8610] Add functional test when two topics are merged
[ticket/8610] Throw an error when the dest topic isn't set
[ticket/8610] Fix bookmarks when merging all the posts of a topic
[ticket/8610] Merging topics does not handle bookmarks correctly