Make CUA-mode use shift-select-mode.

Make CUA-mode use shift-select-mode.
* lisp/emulation/cua-base.el (cua--explicit-region-start)
(cua--last-region-shifted): Remove.
(cua--deactivate): Use deactivate-mark.
(cua--pre-command-handler-1): Don't handle shift-selection.
(cua--post-command-handler-1): Don't change transient-mark-mode.
(cua--select-keymaps): Use region-active-p rather than
cua--explicit-region-start or cua--last-region-shifted.
(cua-mode): Enable shift-select-mode.
* lisp/emulation/cua-rect.el (cua--rectangle-highlight-for-redisplay):
New function.
(redisplay-highlight-region-function): Use it.