Fix `wg-workgroup-buffer-list' docstring
wg-find-buffer: Fix indentation
Implement buffer mapping, wg-switch-to-buffer, wg-kill-buffer

* workgroups.el (wg-ewin->window): Add `buffer' field.
(wg-buffer-mapping): New variable.
(wg-add-buffer-to-workgroup): New command.
(wg-remove-buffer-from-workgroup): New command.
(wg-toggle-buffer-in-workgroup): New command.
(wg-workgroup-buffer-list): Return mapped buffers, not visible
buffers' names.
(wg-buffer-list): Use `eq' since it's buffers not names now.
(wg-find-buffer): Fix because `wg-workgroup-buffer-list' returns
buffers not buffer names now.
(wg-sort-buffers): New function.
(wg-workgroup-visible-buffers): New function.
(wg-buffers-for-switching): New function.
(wg-switch-to-buffer): New command.
(wg-kill-buffer): New command.