MCF format: Simple algorithmic conditionals

Sometimes content deletion is cleaner if entire scenes are removed, but if the scene is a mix of different categories, you might have no way to remove the surrounding time. So some simple combination conditions seem desirable too.

No file, test, or entry point is named. First locate the MCF format parsing and existing scene or category condition handling, then clarify the syntax and expected behavior for combination conditions before implementing tests.

Written by the indexing model from the issue text.