Fix Dict serialization

[RipleyTom]

This fixes the search function after reloading a file that had a dict generated.

stream << it.value()->documents.count(); serialized a qsizetype(u64 on most platforms) but
stream >> numOfDocs; read an int(s32)

This broke reading the serialized Dict.
I removed the need to serialize the size as QT will do it when for us and it is not needed.
I added stream.setVersion( QDataStream::Qt_5_15 ); to both read and write functions to ensure backward compatibility in the future.
I increased DICT_VERSION as it slightly changes the format of the serialization.

[u-235]

Hi. Why not just change the type of the numOfDocs variable? In this case, existing dictionaries would remain valid and would not have to be updated. Adding stream.setVersion is certainly helpful, but the version is incorrect. The minimum supported version is still stated to be Qt 5.5.

[RipleyTom]

If I changed the numOfDocs type it would break the Dict created with QT5 version.
.count() type returning u64 is a change introduced by QT6. I think it is better to have a clean slate.

[gonwan]

QVector<>(size) reserves the size and avoid reallocation, especially for files with large index, I suppose. Better keep it.

[RipleyTom]

The deserializer handles that internally and reserves the space in one go just like we did, why wouldn't it?

[gonwan]

You are right. Misread the code.