(mouse-save-then-kill-delete-region): Take args BEG and END.

(mouse-save-then-kill-delete-region): Take args BEG and END.
Update the undo list properly.
(mouse-save-then-kill): Pass those args.
When we kill, clear mouse-save-then-kill-posn
and mouse-selection-click-count.
(mouse-secondary-save-then-kill): Likewise.
Delete the overlay after deleting the selection text.
(mouse-selection-click-count): Initialize to 0.
(mouse-save-then-kill-delete-region): Handle buffers with no undo list.
Handle empty regions.  Handle end < beg.