Change Bone Dir default value TEMPERANCE to BLENDER

[TokageItLab]

Fixes #1662

As explained there, Bone Dir should keep the original glTF bone orientations by default, so it should use BLENDER. The behavior of not using heuristics by default is same as other importers such as fbx, collada, etc.

I also wrote a slightly more detailed description in ToolTip. I'm not sure if the enum name BLENDER is appropriate to begin with, but we can leave that alone for now.

[CLAassistant]

All committers have signed the CLA.

[julienduroure]

Don't think there is some consensus yet on the "better" default we should have.
So I will close this PR. I don't want to have conversations on both side. Let's continue the discussion on #1662 directly.

I also will have a talk with Blender devs and Khronos Working Group team, to have a feedback.