Instead of having the "Current File" and then the individual libraries
containing indirect library overrides in the Library Overrides display mode,
only show what's in the current file. Agreement was that this isn't very useful
in this view, we may want to add it to the Hierarchy view though (see T95802).

Part of T95802.

Also: The top level ID type items ("Objects", "Materials", ...) are expanded by
default. I personally find this makes things more usable from the get go. E.g.
if you only have objects overridden, there's only a collapsed "Objects" element
and a lot of unused space. Given that in practical situations, probably only a
few ID types would have overrides, only a few elements would have to be
collapsed to clean things up if the list is too long by default. Pretty sure
that is the exeption, not the rule though. Plus, there's the Expand/Collapse
All operator for that.