Emacs mode: delete patches

Teach the emacs mode to delete patches. It will delete the selected
patches, or, if no patches are selected, the patch at point (and if
there is no patch at point, complain). Before deleting, it will ask
for confirmation.

Thanks to David Kågedal <davidk@lysator.liu.se> for lots of elisp
help.

Signed-off-by: Karl Hasselström <kha@treskal.com>