(cua-auto-expand-rectangles): Remove

(cua-auto-expand-rectangles): Remove
automatic rectangle padding feature; replace by non-destructive
virtual rectangle edges feature.
(cua-virtual-rectangle-edges): New defcustom.
(cua-auto-tabify-rectangles): New defcustom.
(cua-paste): If paste into a marked rectangle, insert rectangle at
current column, even if virtual; also paste exactly as many lines
as has been marked (ignore additional lines or add empty lines),
but paste whole source if only one line is marked.
(cua--update-indications): No longer use overwrite-cursor to
indicate rectangle padding