Optimize Kate code completion.

a) Don't resort when we narrowed the filter, as then the sorting
should be preserved. This has a great effect (~50% faster).

b) Don't rematch items whose text did not change. This is mostly
an issue for KDevelop e.g. which has often multiple completion models
per view (word completion, language completion, snippets) and up
until now we would filter all items three times.