Merge mode for annotations-only files

Currently, the ability to unite documents is verified inadequately (see bool MultipleDocumentsReadingModeSelectorController::isAbilityToUniteDocumnents(const QVariantMap& hintsDiocuments)).

If a sequence has too many annotations the "RawDataCheckResult_Sequence" property is not added to it (as only the file header is verified).

Probably the format detection check was done because it was not clear how to merge e.g. GenBank files containing annotations without a sequence.

The annotations-only files should be instead merged as follows: the sequence for the annotations should contain 'N' characters. The check, based on the format detection, should be removed.