[FIX] do not break dataset index (at the wrong moment) when clicking on [Create] row
[FIX] do not break dataset index (at the wrong moment) when clicking on [Create] row

dataset.index was previously set to ``null`` in handler of [Create]
button (to fullfill contract with form view that dataset.index should
be ``null`` to indicate the creation of a new record with no id).

Issue: after setting the index to ``null``, the list view calls
``render_row_as_form`` which starts out trying to save a row being
edited (case: clicking of the [Create] button after having selected a
row for edition or after having written in a new record e.g. [Create]
-> type -> [Create] type -> ...). This tentative to save the existing
form would be performed in the context of a ``null`` dataset.index,
which the form view doesn't (and shouldn't, index should be that of
record *being edited*) expect.

-> first save in whatever dataset state is the current one, and *right
before* creating the new form (after having saved and/or discarded the
previous one) we have the id of the new record to edit (or ``null``),
find the index for *that* and set ``dataset.index`` to that (or
``null``) so the new form view can be created and opened in the right
context.

gisce/openerp-web,gisce/openerp-web,splbio/openerp-web,vnc-biz/openerp-web,vnc-biz/openerp-web,MarkusTeufelberger/openerp-web,splbio/openerp-web,MarkusTeufelberger/openerp-web,gisce/openerp-web,MarkusTeufelberger/openerp-web,splbio/openerp-web,vnc-biz/openerp-web,vnc-biz/openerp-web