Preload sequences

(Depends on #5384)

This forces all sequences to be preloaded on map start. It makes the initial map load longer (roughly 40%) but it prevents the loading freeze in-game.

Note: I initially tried to do it without relying on `OpenWithExts` throwing `FileNotFoundException` but it would require changing too many things or accept `null` values into the `sequences` dictionary, which would be worse long-term. This is why I added `TryOpenWithExts` but don't actually use it.