Storyboards implementation.

[Damnae]

No description provided.

[peppy]

Getting a crash on https://osu.ppy.sh/s/8023:

[Damnae]

Fixed it.

I also implemented loops, but since adding transforms removes transforms on the same property that come later, it probably won't work everywhere...

[Damnae]

Should be ready to be reviewed. I left triggers, samples and integrating it with the game for later, but all storyboards I tested were working (~30).

[peppy]

I'd consider renaming:
StoryboardDefinition → Storyboard
Storyboard → DrawableStoryboard
as this is more in line with the rest of our model/view representations.

[peppy]

looking good

[peppy]

@Damnae Let me know if you're interested in a bounty for this PR. The scale and implementation quality is definitely worthy of compensation.

[Damnae]

@peppy That would be great, thanks!

[peppy]

Awarding $800 in bounty for this. Contact me via email with a paypal account (or osu! supporter tags or whatever if you'd prefer).