[MU4] Update IntervalTree

[Jojo-Schmitz]

The version of IntervalTree we are using has received many upstream updates, however these have not been merged into here since 2013.

Shamelessly stolen from #8808, to have it rebased and be able to check the utests.

[Jojo-Schmitz]

vtests differ in 2 cases, before:

and

After:

and

I don't think those differences are acceptable, loosing quite a bunch of lines, but don't know how this PR might have caused them? Apparently the lines crossing a system break are getting lost.
My guess is that back when PR #8808 had last been touch the vtests were not functional at all, so didn't fail for just that reason.

The utests too seem to fail due to missing lines (slurs, hairpins, etc.).

My backport to 3.x though didn't have those issues as far as I can tell, see 8ef1c5c

[Jojo-Schmitz]

Another rebase, to resolve merge conflicts, no vtest differences anymore (so these weren't the fault of this PR, as suspected).
No unit test failures any more either, so ready for review!

@vpereverzev: mind to review? As you wanted it rebased back in April: #8808 (comment)

[Jojo-Schmitz]

Once again a request to have this reviewed and merged.