Revised draft of 2.0 keybindings

Revised draft of 2.0 keybindings

Thanks to Scott Jaderholm for reminding me that Emacs key binding
conventions prohibit major modes from using C-c <letter>.  Such
sequences are reserved for users.  Therefore, it's likely that the old
keybindings will remain largely intact since it's not clear that they
could be significantly shortened.

As a result, this patch rolls back most of the keybdinding changes in
4930f1c, but not all.  In particular, the org-mode heading navigation
commands were kept, which means that C-c C-p (the physical styles group)
was clobbered.  Use the commands under C-c C-s (logical styles) instead.

The documentation has been updated accordingly.

Summary of changes:

* New outline (heading) navigation commands were introduced:
  C-c C-n, C-c C-p, C-c C-f, C-c C-b, and C-c C-u.
  The previous sequences C-M-n, C-M-p, C-M-f, C-M-b, and C-M-u
  have been restored to their original definitions.

* The following element insertion sequences have been added:
  C-c C-a L (reference links),
  C-c C-t ! (level one Setext heading), and
  C-c C-t @ (level two Setext heading),
  C-c C-a f (footnotes).

* The following element insertion sequences have been deprecated:
  C-c C-a r (reference links),
  C-c C-t t (level one Setext heading), and
  C-c C-t s (level two Setext heading).

* The following element insertion commands have been removed or
  overridden, but have equivalent functionality elsewhere:
  all footnote commands under C-c C-f,
  all physical style insertion commands under C-c C-p.

* Alternative sequences have been added for cases where arrow keys may
  be difficult to use:
  C-c C-x u for M-<up>,
  C-c C-x d for M-<down>,
  C-c C-x l for M-<left>,
  C-c C-x r for M-<right>, and
  C-c C-x m for M-<return>.