.form-actions is no more; .btn-group is not right

.form-actions does not exist in bs3.

.btn-group is used to pack buttons with no intervening space (into a
"button bar".) AFAICT, wrapping a single button in .btn-group is unnecessary.
Update the input group classes for bs3

(DOM nesting was also wrong.)

I am perhaps overly paranoid regarding white space around the input with
the "add-ons".  Though I can not recall details, I do remember having
trouble with this in the past (when using bs2).
Support for .form-horizontal and .form-inline layouts

I'd be all for renaming Form.bootstrap_form_style to something shorter.
(Also, I think it makes more sense as an attribute of FormWidget rather
than of Form.) Deform_bootstrap uses Form.bootstrap_form_style,
however, so compatibility considerations may be warranted.
Merge branch 'form-layouts' into private