Inhibit undo-in-region for mouse-drag-region (bug#37700)

Inhibit undo-in-region for mouse-drag-region (bug#37700)

'mouse-drag-region' leaves the region active around the dragged text,
so a straight undo did not revert the entire operation.  To remedy
this, inhibit undo-in-region when the last command was
mouse-drag-region.  (Method suggested by Stefan Monnier.)

* lisp/mouse.el (undo-drag-region): Set the undo-inhibit-region property.
* lisp/simple.el (undo): Inhibit undo-in-region if the last command
had the undo-inhibit-region property set.
* doc/lispref/symbols.texi (Standard Properties):
* doc/lispref/text.texi (Undo): Document undo-inhibit-region.
* etc/NEWS: Announce the property.