Apply word filter on async completion models
When an async model resets itself in a delayed fashion, we showed
all its items and ignored the word at the current cursor position.
By calling cursorPositionChanged() from the modelReset() slot
in KateCompletionWidget we can ensure that the newly arrived
results are properly filtered too.
This is important for KDevelop where the clang provided completion items arrive with a noticeable delay. Something changed in handling those items in the last few months, breaking the filtering. Now the async results are correctly filtered again and the new unit test coverage should hopefully prevent further breakage in the future.