Also improved error logging, forced the number of errors to display when the pack is loaded starting, and added stricter checks when using hd pack version 109+
Notes:
-Uses a copy of VirtualizingStackPanel (to include fixes done after preview6) + fix another issue in lists
-Fixes memory leaks when closing windows (by setting DataContext to null, etc.)
Keep BGs in separate arrays based on their priority to avoid having to scan thousands of background entries for layers that have no backgrounds at all on each frame
