Fix the behavior of C-a when visible-mode is active

Fix the behavior of C-a when visible-mode is active

* lisp/org.el (org-truely-invisible-p): New function.
(org-beginning-of-line): Use `org-truely-invisible-p'.

Sebastien Vauban writes:

> I'm reporting some movement problem I've observed. When on any line, let's say
> the last clock one:
>
> - `C-e' moves the cursor to the end of it
> - `C-a' moves the cursor at the level of `LOGBOOK'.
>
> --8<---------------cut here---------------start------------->8---
> **** Emails and News
>     :PROPERTIES:
>     :ID:       62849012-ae7a-4885-a552-b34516d3fd06
>     :Effort:   1:00
>     :END:
>     :LOGBOOK:
>     CLOCK: [2010-06-01 Tue 08:35]--[2010-06-01 Tue 09:20] =>  0:45
>     CLOCK: [2010-06-02 Wed 09:10]--[2010-06-02 Wed 10:20] =>  1:10
>     CLOCK: [2010-06-03 Thu 09:25]--[2010-06-03 Thu 10:20] =>  0:55
>     CLOCK: [2010-06-04 Fri 09:00]--[2010-06-04 Fri 10:00] =>  1:00
>     CLOCK: [2010-06-07 Mon 09:05]--[2010-06-07 Mon 10:25] =>  1:20
>     CLOCK: [2010-06-08 Tue 09:05]--[2010-06-08 Tue 10:30] =>  1:25
>     CLOCK: [2010-06-09 Wed 09:05]--[2010-06-09 Wed 10:20] =>  1:15
>     CLOCK: [2010-06-10 Thu 09:05]--[2010-06-10 Thu 10:20] =>  1:15
>     CLOCK: [2010-06-11 Fri 09:15]--[2010-06-11 Fri 10:00] =>  0:45
>     CLOCK: [2010-06-14 Mon 09:15]--[2010-06-14 Mon 10:10] =>  0:55
>     :END:
> --8<---------------cut here---------------end--------------->8---
>
> This is -- at least -- true whenever `M-x visible-mode' is enabled.