Support tracking the `bone_collection.is_visible` property via library
overrides.
This requires an RNA change. Instead of exposing all bone collections as
`armature.collections.all` (i.e. a sub-property of `.collections`) it is
now exposed as `armature.collections_all` (an armature property).
There is still the limitation that insertion operations are only
supported on `armature.collections`, so only new roots + their sub-trees
can be added via library overrides.
Overrides on `armature.collections_all` are limited to overriding
property values. The `parent`, `index`, and `child_number` properties
are excluded from this, as reorganising the hierarchy itself via
overrides is not possible.
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