LibOverride: Disable the re-parenting of overriding objects to the instancing empty.

Does not seems to be that useful... And it's breaking the objects also
'parented' to an armature through a modifier...

LibOverride: Fixed bug in removing override op operator.

In case the property is a RNA pointer, `RNA_path_resolve()` will try to
resolve it and return that pointer, instead of returning expected
container... That is a bad inconsistency in the rNA path API, but no
proper way to solve it for now...

Merge branch 'master' into asset-engine

Conflicts:
source/blender/blenloader/intern/versioning_280.c

LibOverride: Add some basic non-nodal material properties to override realm.

Revert "Fix T68971: Copy As New Driver from Material node creates a bad reference."

This reverts commits 54fd8176d7e91, 4c5becb6b1 and 8f578150e.

Those kind of commits must be reviewed and approved by project owners.

That one:
* Broke Collada building by not properly updating all calls to modified
function.
* Broke *whole* ID management by not properly updating library_query.c.

And in general, I am strongly against backward ID pointers, those are
*always* a serious PITA for ID management. Sometimes they cannot be
avoided, but in general other ways to get that kind of info should be
investigated first.

Cleanup: Split some code out of rna_access.c

That file was getting out of control, now comparison/override RNA code is
in `rna_access_compare_override.c`. 1K lines of code for now, but that
area is likely to grow more in the future...

Note that we can probably split more out of `rna_access.c`, but for now
that will do.

LibOverride: Enforce full override operations check on file save.

We try to avoid diffing too much things during edition, but when saving
a file we need to check all possible overridable IDs to ensure we have
all needed override operations...

Was pretty sure that was already in code, but apparently not (or it got
lost at some point...).

LibOverride: Fix broken logic in code checking and adding new override ops.

When we wanted to force all overidable IDs to be checked, code would end
up checking the whole Main DB, instead of only overrideable ones.

LibOverride: Cleanup: tget rid of G.main usage when we can get it from context.

LibOverride: Cleanup: rename parameters in RNA apply code.

We cannot use local/reference here, that is very confusing, since at
that stage current local is kind of src of data for the future local ID,
that is currently a mere copy of the linked data... ;)

So we are much better with src/dst names here.