(mouse-move-drag-overlay): New function.

(mouse-move-drag-overlay): New function.
(mouse-drag-region-1): Use it.
Try to simplify a bit the state handling.  Handle clicks on links
inside intangible areas.
(mouse-save-then-kill): Minor simplification.
(mouse-secondary-overlay): Make it always non-nil instead of
recreating it each time.
(mouse-start-secondary, mouse-set-secondary, mouse-drag-secondary)
(mouse-kill-secondary, mouse-secondary-save-then-kill):
Simplify accordingly.