Rework beatmap conversion process to use IBeatmap

[smoogipoo]

Prereqs:

• #2490

Significant changes:

• WorkingBeatmap now exposes a GetPlayableBeatmap(RulesetInfo) method, which returns the converted beatmap with all mods applied. Previously this was done in RulesetContainer.
• No more object-specific mods. e.g. IApplicableToHitObject<TObject> is now IApplicableToObject.
• BeatmapConverter and BeatmapProcessor now take an IBeatmap constructor parameter, because some of them typically need to determine if the beatmap passed in is for the current ruleset or not, prior to conversion actually happening.

Currently broken:

• Difficulty calculation - star difficulty is computed before a WorkingBeatmap becomes available during the import process. This process is wrong anyway.

[peppy]

This is a pretty hard change to review, but everything seems to be in the right place. Let's go for it 👍 .