Modified algorithm to remove duplicated suggestions

Bug 5916225

Duplicates were removed when received from SpellChecker, in a way
that could move the top candidates lower in the list.

Moved that code to the part that creates the actual suggestion list,
to make it more generic. The order of the first SuggestionSpan is
guaranteed to be respected.

Also mentionned non null suggestions and fixed a problem in SuggestionSpan
constructor.

Change-Id: Iaa3b1b84ae512451e439e5c5e63448c2a19145b5