Do not update the square all the time while panning

[HaonRekcef]

This change is needed for lichess-org/mobile#2213
This will allow users to delete a piece by pressing the square again.

[veloce]

Not sure about the consequences of that one. Panning is also needed for the editor so we can add same piece by panning to multiple square during same gesture. We don't want to remove that ability.

[HaonRekcef]

I should have been more clear. This is exactly why we need these changes to chessground and lichess-org/mobile#2213 is not sufficient. Here a video recording:

board_editor.webm

One possible issue with this implementation is that if you go over a square twice while panning, it will remove the piece again. This could be fixed, but at the same time panning is not that useful anyway, since in normal chess there are at most 8 of the same piece on the board.

[veloce]

Useful for the pawns. I think it is fine like that, but will look at it in more detail. Thanks!

[veloce]

I guess we could add a simple test here to show effect of the change.

[HaonRekcef]

Added onPanCancel, good catch.
Not really sure what to test, the only thing that changed is that while panning onEditedSquare is only called once per square. I have included a test case in the lichess-org/mobile#2213 PR though.