Fix tree-view item ignoring custom matches() method override

No user visible change expected (this isn't used in existing code yet).

Users of the tree-view API should be able to write their own `matches()`
method to compare tree-view elements after tree reconstruction. Part of
the tree-view matching process wouldn't use this though, and use the
`matches_single()` method instead, causing issues like multiple items
sharing the same state. This was initially done for an optimization
(the default `matches()` compares parents as well, which seems redundant
here), but it backfires and actually isn't needed. The default
`matches()` only compares the parents when `matches_single()` returns
true anyway, so the redundancy is really minor and not a performance
concern.

source/blender/editors/interface/views/tree_view.cc

@@ -108,7 +108,7 @@ AbstractTreeViewItem *AbstractTreeView::find_matching_child(
     const AbstractTreeViewItem &lookup_item, const TreeViewOrItem &items)
 {
   for (const auto &iter_item : items.children_) {
-    if (lookup_item.matches_single(*iter_item)) {
+    if (lookup_item.matches(*iter_item)) {
       /* We have a matching item! */
       return iter_item.get();
     }