-Also allows cropping the edit window when the size doesn't fit (e.g using 4x4 near the bottom of the screen can result in a 4x2 edit window, etc.)
-Fixed a display issue when the number of edited columns & rows didn't match
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.)