Use revert-buffer-function instead of my revert-buffer-dwim.

Seems more appropriate to use a buffer-local revert-buffer-function for reverting temp buffers. So:

 - Ditch my/revert-buffer-dwim
 - Have my/yank-clipboard-to-temp set the local revert function, and mark the buffer unmodified.
 - Update my/undo-all to be a valid implementation of revert-buffer-function.
For multiple cursors, use the more convenient mc/mark-all-like-this-dwim.