org-indent: implement asynchronous initialization
* lisp/org-indent.el (org-indent-initial-marker,
org-indent-initial-timer, org-indent-initial-lock): new variables.
(org-indent-mode): at initialization, start an idle timer to indent
the whole buffer. When the user is asking for control, interrupt the
process, and resume at the same point when idle again.
(org-indent-initialize-buffer): new function.
(org-indent-add-properties): throw an interrupt when indentation of
buffer is stopped during initialization.