LibOverride: Do not delete no-more-used overrides during resync if they are user-edited.

Ultimately those will be listed with a special icon in the upconning
Outliner overrides view.

LibOverride: auto-run resync process on file reading.

LibOverride: Add second part of auto-resync code.

`BKE_lib_override_library_main_resync` uses
`LIB_TAG_LIB_OVERRIDE_NEED_RESYNC` tags set by RNA override apply code,
and perform detection for the remaining cases (those were new overrides
need to be created for data that was not present before in the library).

And then it actually resync all needed local overrides.

LibOverride: First stage of detection of 'need resync'.

We can fairly easily detect some resync-needed cases when applying the
overrides operations on a Pointer RNA property.

This should cover all cases where an existing override's ID pointer is
changed in its linked data.

We still have to add code to detect when a not-yet-overridden linked ID
needs to become overridden (because its relations to other data-blocks
changed in a way that requires it).

LibOverride: Add a utils to check if override has been user-edited.