[FIX] @groups attribute should not disappear when editing

[FIX] @groups attribute should not disappear when editing

The @groups attribute in qweb views was not rendered (even when matched by the
user), so editing a template with an @groups would either remove the whole
section (if the user didn't have the groups, fixed in previous commit) or only
removed the attribute itself making it visible to everybody (which ought be
fixed-ish by this commit).