Cache reference movies for idle-imported formats

[astrange]

After loading an mkv/flv/etc., the reference movie should be saved in a user cache directory, so it can be opened immediately instead of parsing the file.

Problems:

• #148 needs to be done first (or else we still need to read mkv attachments)

• We need to be sure the cache isn't out of date - this is problematic because hashing the original file would mean reading it. Checking the mkv uuid and file size would work for mkv, is reading the mod date good enough for anything else?

[gbooker]

Many synchronization systems, such as rsync, check the file size and mod date to determine if the file has changed. They only hash the actual file if explicitly told to do so. I'd say that size/mod date is sufficient.