Provides searching functionality for folders.
{@link DefaultSearchEngine} is using filter plugins to perform search
requests.
Every {@link QueryEngine} implementation supports a subset of search
requests, for example searching in the body or subject of a message.
{@link DefaultSearchEngine} is also the fall-back for every search request
which is not performed by implementations of {@link QueryEngine}.
We use composition to plug the {@link QueryEngine} in the
{@link DefaultSearchEngine}, using a {@link DummyQueryEngine} which
doesn't support any special search requests, as default.
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