Revert TrackerResetsHandler & add comments

[ButterscotchV]

My changes to the euler order in #1595 probably messed things up quite bad for some people. I originally thought I messed things up big time, but I think it's mainly just the change to the euler order, where I made it so roll no longer affected yaw estimation, but pitch did... I'm guessing people just have their trackers tilted too far forward/backward.

Manual mounting orientation being applied after attachmentFix may also be a source of problems, since a change in one variable will affect the other.

tl;dr: This will revert to the original behaviour but with acceleration fixed.

[Eirenliel]

The reason that the changes I made worked at all seems to be because official SlimeVR trackers have a correct IMU rotation applied, so the actual value of attachmentFix will be approximately equal to the quaternion identity, therefore the heading correction will be approximately equal between IMU and bone spaces.

Except Butterflies don't and we routinely use them upside-down.

[Eirenliel]

Also maybe let's give all the corrections better names?