No functional changes.

As indicated in D12912, it is important to change the drawing that follows
the cursor during dragging.

The solution in that patch was to create the parameter drag->no_preview
which is set in the pool function and when true ignores part of the
drawing.

But that abuses the pool function and is not enough to setup the final
drawing.

This patch proposes to create 3 callbacks for wmDropBox:

void (*draw)(struct bContext *C, struct wmWindow *win, struct wmDrag *drag, const int xy[2]);
void (*draw_activate)(struct wmDrag *drag);
void (*draw_deactivate)(struct wmDrag *drag);

• draw: overwrites the default drawing
• draw_activate: called when a drag item activates the dropbox the first time
• draw_deactivate: called when the dropbox of a drag item changes or when the drag item is freed.