Fix Outliner highlighting multiple base elements in different libraries

In the Blender File display mode of the Outliner, mouse hovering a
"base" element (e.g. "Objects", "Materials", ...) would also highlight
that same base element in other libraries linked into the scene. In fact
operations like (un)collapsing would be applied to both too.
Issue was that we'd always use the listbase containing the data-blocks
from the current main as a way to identify the tree element. So for the
same data-block types we'd use the same listbase pointers. Instead use
the the library pointer + a per library index.

source/blender/editors/space_outliner/tree/tree_display_libraries.cc

@@ -150,7 +150,7 @@ TreeElement *TreeDisplayLibraries::add_library_contents(Main &mainvar, ListBase
         }
         else {
           ten = outliner_add_element(
-              &space_outliner_, &tenlib->subtree, lbarray[a], nullptr, TSE_ID_BASE, 0);
+              &space_outliner_, &tenlib->subtree, lib, nullptr, TSE_ID_BASE, a);
           ten->directdata = lbarray[a];
           ten->name = outliner_idcode_to_plural(GS(id->name));
         }