Change the storage of keyframes from a std::vector to std::unordered_map

[puzzlepaint]

Currently, the keyframes are stored in a std::vector, and removing keyframes leaves nullptr entries in that vector. This is likely not to be handled correctly in all parts of the codebase, leading to crashes. It would most likely be preferable to store the keyframes in an unordered_map instead (mapping keyframe_id -> keyframe_pointer) and delete removed keyframes from it entirely.

[puzzlepaint]

Hi, this comment is unrelated to the original topic of this issue. I thus moved it to a new issue: #16