Add the option to remove the shadow for dragged pieces

[EmmetSchuler]

It seems that some users on the forum dislike the shadow that appears when they drag pieces, because they are used to dragging on the website without one.
As we already have a toggle for piece magnification, I think this is relevant to certain users.

[anthisse]

When you mean shadow, do you mean the round circle that appears when dragging the piece over another square? Or instead do you mean the translucent piece that is displayed in the original square when dragging the piece?

[EmmetSchuler]

I'm referring to the grey circle under the dragged piece

[anthisse]

Opening a pull request now; I will start to work on this.

[ijm8710]

This just feels like people being way too uptight. Feels unnecessary imo

[EmmetSchuler]

Maybe not the most necessary, that's true. But a lot of people are coming from the website where there isn't a shadow. Someone made a feedback post about it so I guess there's at least a userbase for it.

[anthisse]

Upon further inspection, it seems that the circle comes from flutter-chessground; it's being drawn by _DragAvatar in board.dart here: https://github.com/lichess-org/flutter-chessground/blob/25de90896c8e00a825efaf174850d5b44a9c9048/lib/src/widgets/board.dart#L781

It doesn't appear to have an interface to turn it off unfortunately. To add the functionality, another bool (perhaps called showPieceShadow) inside of ChessboardSettings needs be added with associated mutators and accessors to set _DragAvatar::decoration to always be null when false. I'll submit a pull request there to add that functionality.