[o2m] o2m and buttons

The little demonstration module published in this bug report https://bugs.launchpad.net/openerp-web/+bug/965326 , is surprisingly good to make appear strange bugs in o2ms.

Two bugs I could identify:
- Create a "First class" record, do not save it, click to a create a "Second class" record in the o2m. Now click on the test button -> the button become grey and does not come back to normal. Nothing else happens.
- Create a "First class" record, do not save it, click to a create a "Second class" record in the o2m. Save the "Second class" record (do *not* save the "First class" record). Re-open the "Second class" record, click on the test button -> exception.