ReaderDictionary: merge dict & dict_ext results (#8523)
ReaderDictionary: merge dict & dict_ext results (#8523)

So the presence of a dict_ext and results from it
are transparent to callers.
(This fixes the warning log when dict_ext is there.)
poire-z/koreader,Frenzie/koreader,NiLuJe/koreader,poire-z/koreader,Frenzie/koreader,koreader/koreader,koreader/koreader,NiLuJe/koreader