[ADD] tentative removal of attachments

[ADD] tentative removal of attachments

Attachments ought be removed only if they are not used in an existing page/web
view.

Theoretically this could be set directly in unlink(), but:
* that would make a nice error message significantly harder
* the expenses of performing a text search in all view archs would be a bit expensive

Notes:
* the views set could be reduced to only "web" views
* the search is likely sensible to false negatives e.g. different order of
  query parameters. It *will* remove images still being used.

bzr revid: xmo@openerp.com-20140304110716-u14w6uo8fbkfa42i