Use a stable sort for hitobjects

[smoogipoo]

Would change the order of hitobjects which would change the output of mania beatmap conversion.

A stable sort is intended here, see: https://github.com/peppy/osu-stable/blob/master/osu!/GameplayElements/HitObjectManager.cs#L822-L832

Brings osu!lazer and osu!stable diffcalc within 1 d.p. of each other:

Beatmap	Mods	Expected (strain)	Actual (strain)
1023967	None	7.56702	7.56223
1023967	DT	10.4857	10.4810

Further accuracy isn't really possible due to osu!stable doing an unstable sort with a different algorithm than what .NET Core does, just before difficulty calculation.

[peppy]

Let's add a comment explaining what we consider to be stable sorting here.

[smoogipoo]

On a further look inside mania beatmap conversion, I've come to the conclusion that this ordering doesn't really matter for mania-specific beamtaps since no RNG is done, and non-mania-specific beatmaps generally don't have multiple hitobjects at the same time, so I'm going to hold off on this for now in favour of less memory usage/faster sorting.

[smoogipoo]

Nope lol this totally matters in mania diff calculation :p

[smoogipoo]

Hold off on this for a bit, there may be some sorting applied elsewhere that affects objects further in diff calculation.