[FIX] web_editor: visibility of customize snippet option button

Commit 57c09e8a was supposed to fix the problem but only made it
worse in some cases. Since the new option parenting system, the
customize button visibility should not be determined on creation as
parent and own options are appended on snippet focus. Also, removing
the dropdown header if it was alone was a bad idea as this is it which
contains the information of options and all the options have not
necessarily a <li/> tag in the dropdown.

For example, the many2one option has no <li/> in the dropdown and is
the only option of the field snippet. So with the previous commit, the
dropdown header was removed as there is no <li/> in the dropdown other
than this header. So as the header was removed, on snippet focus, there
was no way to determine the many2one option exists, so its on_focus
method was not called.

Now, the dropdown-header is always kept. The visibility of customize
button is determine on snippet focus.