* lisp/emulation/cua-base.el (cua-paste): Add `delete-selection' property

* lisp/emulation/cua-base.el (cua-paste): Add `delete-selection' property
instead of deleting the selection "by hand".
Rely on insert-for-yank to yank rectangles.
(cua-highlight-region-shift-only): Mark obsolete.
(cua-mode): Don't enable/disable transient-mark-mode,
shift-select-mode (cua-mode works both with and without them), and
pc-selection-mode (obsolete).
* lisp/emulation/cua-rect.el (cua--activate-rectangle): Activate the mark.
(cua--deactivate-rectangle): Deactivate it.