Added support for `revert-buffer' (*mu4e-main* buffer)

The `mu4e~main-view' command now uses `revert-buffer' and does not
switch to the *mu4e-main* buffer. The old code of `mu4e~main-view'
is now in `mu4e:main-revert-buffer'.

Now mu4e~main-toggle-mail-sending-mode can easily be called from other
mu4e buffers without switching to the main buffer first, one just
needs to add the desired keybindings to the desired keymaps (see the
commented example at the end of mu4e-main.el).

All changes are preceded by a "NEW" comment for easy location.