Improved and unified the folder drag and drop validation in the Content Browser
UE-9025 - User gets green check mark when trying to move class into content folder
The Content Browser folder drag and drop validation had not been updated to handle class folders. This meant that it was possible to crash the editor by dragging class folders around, and also meant that you were given misinformation when trying to copy a class around.
I've unified all the duplicated logic from SPathView, SAssetView, SAssetTreeItem, and SAssetViewItem into utility functions inside a new DragDropHandler in the Content Browser. This class correctly validates that you can only interact with assets and asset paths, but doesn't outright prevent you being able to include classes and class paths in your drag operations (it just warns you about them, and the strips them out of the drop lists).
[CL 2438229 by Matthew Griffin in Main branch]
