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.
Update embeded documentation.

* helm-help.el: Update string for helm-find-files.

* helm-files.el: Update url's in comments.
Fix details on remote candidates (#749).

* helm-buffers.el (helm-buffer--details): Do it.
Get file actions with (helm-actions-from-type-file).
Get rid of persistent-help plugin (#757).

* helm-source.el (helm-source): Don't plug persistent-help.
(helm-source-sync,
helm-source-in-buffer): same.
(helm-source--header-line,
helm-source--persistent-help-string): new.
(helm--setup-source): Use them.
* helm-source.el (helm-source): Initialize header-line to nil.
Merge pull request #761 from emacs-helm/pr-get-file-actions

Get file actions with (helm-actions-from-type-file).
Merge branch 'exp'
fix #746

Use `add-face-text-property' instead of `put-text-property' to highlight
matches so that faces in candidates are not just erased