Reworking datasets

[AlekseySh]

# We will remove using keys in __getitem__ for:
# Passing extra information (like categories or sequence id) -> we will use .extra_data instead
# Modality related info (like bboxes or paths) -> they may only exist as internals of the datasets
# is_query_key, is_gallery_key -> get_query_ids() and get_gallery_ids() methods
# Before this, we temporary keep both approaches to make migration easier

• Introduced slightly different interfaces for datasets
• Made a dedicated folder for images datasets. Renamed and moved old datasets. Kept names of the old dataset for backward compatibility.
• Removed ListDataset and its' tests
• tests: DummyQGDataset -> EmbeddingsQueryGalleryDataset

• update docs
• QG and QGL datasets for both: embeddings and images