org-odt.el: Put table width under user-control

org-odt.el: Put table width under user-control

* lisp/org-odt.el (org-odt-table-style-format): New.  Template for
auto-generated table styles.
(org-odt-automatic-styles, org-odt-object-counters): New
variables.
(org-odt-add-automatic-style): New function.
(org-odt-write-automatic-styles): New function.  Create
automatic styles for tables that have custom :rel-width.
(org-odt-begin-table): Parse attributes specified with
"#+ATTR_ODT: " option and use it to create an automatic table
style.
(org-odt-save-as-outfile): Call
`org-odt-add-write-automatic-styles'.
(org-odt-init-outfile): Init newly add variables.
(org-odt-section-count): Remove it.
(org-odt-begin-section): Use `org-odt-add-automatic-style' to
generate an automatic section name.

Customize table width using :rel-width option.  For example,
to create a table of width 60% use:

    #+attr_odt: :rel-width 60
    | A | B |
    |---+---|
    |   |   |