Fix T70739: Make Library Override doesn't re-target Armature constraints.

Missing one step (collection prop itself) in RNA hierarchy of properties
leading to those armature object pointers...

Node shader wrapper: use 'Non-Color' profile for BW textures inputs.

All the single-value texture inputs of Principled BSDF node should use
non-color colorspace profile, not sRGB one (issue raised in
https://blender.stackexchange.com/questions/155617, thanks).

That also revealed another issue - since those color space settings are
stored at the image level itself, not the node one, we need to
duplicate those image data-blocks when we use same picture for e.g. base
color (sRGB) and specular (non-color) inputs...

For now using a basic mechanism for that, might generate several extra,
uneeded copies of the image ID, but that?s better than breaking custom
settings and such.

Note that while this will modify the behavior of the impporters using
that node wrapper, no change should be needed in IO add-ons themselves.

Fix T70787: Duplicating objects with custom property of type ID creates bogus links.

Note that the issue also affected animdata handling...

Checked all usages of the `BKE_libblock_copy_ex()` function, and
think never handling user count here is valid, although a bit risky
maybe. But other solution would be to add yet another copy flag, so
would rather go with that one for now.

Update Py API link in help menu to point to version dir also in beta.

Now that we have beta and master in parallel, we cannot point to API doc
from master in beta builds of incoming release anymore.

Py API doc generator: tweak to handle new release process.

We need to handle beta stage in a specific way, since it's no longer
master, but not yet 'real' rc/release stage...

For now, only point to version dir of the API doc, but no need to create
any symlink (that way, 'current' remain pointing to 2.80 release, while
'2.81' is no longer a symlink to 'master', but its own actual doc).

Py API doc generator: more tweaks to adapt to new release process.

Now we also create a 'version' symlink for master.

Fix T70695: Scene crashes Blender on open.

Note that this commit fixes the crash itself, but actual issue is *how*
that situation could happen (having insert override operation with local
'source' overriding data-block with an empty bone constraints stack...).

Fix T70714: Crash when using OBJECT_OT_material_slot_assign op without UI.

There is no guarantee we can get any valid UI from the context, and that
operator can work without it.

Fix T70596: Wrong default scale value for node wrapper tool.

This affected all exporter add-ons using that wrapper to handle the node
shaders... sigh...

Node Shader wrapper: add access to the Emission color of Principled BSDF node.

This did not exist when that wrapper was created, but is supported by
many formats, so worth supporting it now. See also T70666.