CHANGELOG: look for dicts in /usr/share/hunspell by default, this is a hunspell plugin, right?

CHANGELOG: add compile-time check to use /usr/share/myspell if /usr/share/hunspell doesn't exist
CHANGELOG: fix Highlighter::setCurrentLanguage() for the case when previous language was invalid
CCBUG: 349151

note: i had spellcheck broken on my machine because i had installed myspell-ru which uses KOI8-R encoding,
while hunspell-ru uses UTF-8 as expected