Get non-primary image handles

[jwuensche]

Seems like an useful addition. Recently also found this feature missing, and having it upstream on crates.io would be nice for any future users.

Thanks to all of you for developing this crate btw :smile:

[Cykooz]

I have released new version of libheif-rs. Thank you for your suggestion.

[0.14.0] - 2021-03-17

• Added new methods: ImageHandle::depth_image_ids(), ImageHandle::thumbnail_ids(), ImageHandle::metadata_block_ids().
• Deprecated some methods: ImageHandle::list_of_depth_image_ids(), ImageHandle::list_of_thumbnail_ids(), ImageHandle::list_of_metadata_block_ids().
• Added new methods for getting top level images from HeifContext: HeifContext::top_level_image_ids(), HeifContext::image_handle().