FIRE-36479 Probable fix for nested scrollbars in outfits panel

LLFlatListView::sort() rearranged items via rearrangeItems(), but never notified the parent accordion tab about the updated content height.
When outfit items were loaded without an active filter this notification was skipped entirely, so that left the flat list stuck with an outdated size. Because LLFlatListView inherits from LLScrollContainer, it ended up showing its own scrollbar inside the accordion's scrollbar - hopefully fix it by calling notifyParentItemsRectChanged() after rearrangeItems() in sort(). The call is guarded by mPrevNotifyParentRect, so redundant calls should be safe.
Now... would it scale with thousands of outfits that some residents have? Let's see in the nightlies...

Signed-off-by: PanteraPolnocy <panterapolnocy@gmail.com>

indra/llui/llflatlistview.cpp

@@ -443,6 +443,7 @@ void LLFlatListView::sort()
 
     mItemPairs.sort(ComparatorAdaptor(*mItemComparator));
     rearrangeItems();
+    notifyParentItemsRectChanged(); // <FS:PP> FIRE-36479 Multiple nested scrollbars in Appearance window
 }
 
 bool LLFlatListView::updateValue(const LLSD& old_value, const LLSD& new_value)