Editor improvements: dragging scenes to viewport, undo for gizmos, better docs

[DP News-Robot]

You can now drag scene files (like glTF, X3D, images…) from the “Files” bottom panel onto a viewport. It will automatically create a TCastleScene instance with proper URL that displays this model. The initial translation is determined by the point where you drop the item, with some smart code that should account for both typical …