Improve helm-walk-directory.

* helm-utils.el (helm-walk-directory): No need to check for symlinks
with stringp as type=t ensure dir is not a symlink.
The helm-walk-directory version using file-name-all-completions.

* helm-utils.el (helm-walk-directory): Use file-name-all-completions.
* helm-utils.el (helm-walk-directory): Fix long lines.
* helm-utils.el (helm-walk-directory): Fix bug with infinite loop on symlinks.
* helm-utils.el (helm-walk-directory): Call directory-file-name only once.
Keep improving helm-walk-directory.

* helm-utils.el (helm-walk-directory): Even faster not computing the dotted
files at start of loop but one by one.
Allow fontifying helm-org-headings.

* helm-org.el (helm-org-headings-fontify): New user var.
(helm-get-org-candidates-in-file,
helm-org-get-candidates): Use it.
Add a new command for all sources documentation.

* helm-help.el (helm-documentation): New command.
(helm-documentation-file): New user var.
(helm-help--string-list): new internal.
* helm-aliases.el: Add helm-org-headlines alias.
Fix expansion of tilde in tramp names (#753) (#747).

* helm-files.el (helm-ff-set-pattern): Add a special rule for tramp names with tilde.
* helm-files.el (helm-browse-project-find-files): Fix long lines.
Improve helm-walk-directory directory checking.

* helm-utils.el (helm-walk-directory): Add comments,
make checking of directory a little faster.