Handler.Box: Maintain the style of the cursor when documentDrag=true.

[jorix]

Dragging the corner of the box out the map (when documentDrag=true) the cursor is displayed as "move" instead of "crosshair"

To correct this, the patch proposes that only Handler.Drag makes the management of the cursor style using a new APIProperty cursorDragStyle. (this idea + CSS can be applied also to correct the same problem in Control.DragFeature)

Tests are added.

NOTE: It may be interesting to apply this ticket in 2.11 If #3363 is applied in 2.11

[ahocevar]

Looks like a solid patch at first glance. Should definitely be considered for 2.11 if #3363 is pulled.

[jorix]

NOTE: If #3363 is applied the tests in this patch should be adjusted.

[jorix]

See also #3372