Allow layered ScrollContainers to coexist on input handling

[frenzibyte]

• Closes ScrollContainers of different directions should be able to coexist #3576

This changes drag event handling and scroll event handling in ScrollContainers to be based on their ScrollDirection, to allow for layered scroll containers to coexist.

This however becomes a nuisance on two parts:

• 20c1d72: In order for scroll containers to coexist, the innermost must not OnMouseDown to let the parenting scroll containers enter the mouse-down/drag input queue, for receiving drag events.

  I'm not entirely sure if there's a reason for that in the first place, checking with git blame threw me back to 4 years ago (General fixes. #316).

• 7bff9b9: When a scroll container is horizontal, scrolling vertically will also be considered as horizontal, therefore this coexistence change cannot apply when the inner scroll container is horizontal (as done in test).

  In addition, it also results in this kinda weird UX (inner = horizontal, outer = vertical):

  CleanShot.2022-04-22.at.06.47.29.mp4

[peppy]

Haven't tested yet, just reading code changes

[peppy]

Will need to check osu!-side to make sure nothing was relying on this behaviour, but I'm not strongly against this change if it works everywhere we expect it to.

[frenzibyte]

I would be interested to see if there's any scroll container overlayed on top of another component at all (unless wrapped in an OverlayContainer of course).

[frenzibyte]

Unsure how to deal with this test:

Feels like an arbitrary choice to drag perfectly diagonally and check whether the scroll container would block the click or not, since it's going to depend on which axis is checked first in the early-guard.

[peppy]

The diagonal in that test does look arbitrary. Changing to a specific direction is probably fine.

[peppy]

Made some minor refactors, but looks and feels good to me. Will wait for a second opinion for merge.