Fix hover event

Fix hover event

HoverEvent was relying on highlight-dragging to detect if the mouse pointer
entered a new square, but on XBoard highlight dragging is sick. Now the
routine remembers the previous mouse position independent of the highlights.
To detect if we are in the starting square, it looks for change of the
from-square, in which case it fakes (first) entry of the current square
(where it saves marker state, so it can be restored after we leave the
square again, in case the engine saw fit to respond to the hover command).