Dragging operations are now encapsulated into special inner classes (`StampTool.TokenDragOp` and
`PointerTool.TokenDragOp`) which track all the stack needed for a drag operation. These classes also give a convenient
place to store some calculations that are necessary for dragging to work properly with modified token layouts.

The key difference in dragging now vs before is that a drag is a displacement from a starting `ZonePoint` to a
destination `ZonePoint`, which reflects the motion of the cursor. This displacement is then applied to the token's drag
anchor (formerly called drag offset) to decide where to move the token. Snap-to-grid is applied at the end of the drag
if needed. This enables smooth dragging to work without "hiccups", and allows the same for snap dragging when the token
layout has been shifted.

`SelectionSet` has been modified to accept the final point, rather than trying to work with offsets. The offset approach
pretty well required every component to keep track of which token is moving from where in order to figure out the same
thing: where should it end up. Now the caller can just say where the token should end up.

Similar to token dragging, the free resize of stamps had a similar "hiccup" bug that has been fixed. Resizes are now
encapsulated in `StampTool.TokenResizeOp` and works correctly whether or not the token is snap-to-grid, allows for
aspect ratio to be preserved for rotated tokens, and respects token layout.

To help with diagnosing issues during drags, new debug rendering was added to `PointerTool` and `StampTool` with a new
developer option to enable it.