I think maybe we should assign C-c C-c to occur-edit-mode instead.

   1. After C-x C-q, I can no longer kill entire lines in the occur buffer.
   Trying to do so reports "Text is read-only".

The text properties are now added when entering occur-edit-mode.

   2. After C-x C-q, If I delete some text in the occur buffer, then use
   "undo", when I reach the point at which there is no more to undo, I get:
   "Wrong type argument: markerp, nil" rather than "No further undo information".

   Debugger entered--Lisp error: (wrong-type-argument markerp nil)
     marker-buffer(nil)
     occur-after-change-function(1 40 39)
     primitive-undo(1 ((nil font-lock-face underline 1 . 40) (t 0 . 0)))
     undo-more(1)
     undo(nil)
     call-interactively(undo nil nil)

Fixed.

   3. After C-x C-q, M-x revert-buffer in the occur buffer triggers an error

occur-revert-arguments is killed after any major-mode change. In the
patch this has been made permanent-local.

Comments welcome. Thanks.