Add test and change the function description

Bugfix

In D23061#509851, @cullmann wrote:

Hmm, I think we want an unit test for this, too.
And I think it won't work like it is, before the call to initLookupForCaseSensitivity one needs to clear both m_keywordsSortedCaseSensitive and m_keywordsSortedCaseInsensitive vectors.

Use setKeywordList instead previous API

In D23061#509540, @cullmann wrote:

Actually, wouldn't it make more sense to just provide a way to set the full keyword list?
You can get the QStringlist will all keywords already with keywordList, a setKeywordList(...) would make more sense to me than this modifiers.
That would allow to alter it like you want and then write it back in one go with just one re-init of the mapping.