AUI-1248 Allow users to request buttons to not have the btn-default class

Some buttons, like the close button, shouldn't have the btn-default class. This adds an attribute to buttons so users can specify when they don't want this css class to be added to their button.
AUI-1251 Fix FormBuilder edit buttons alignment on Bootstrap 3

Adds min height to serve as a clearance for making space for the form builder toolbar action buttons.
Update toolbar class name to use the css prefix 'btn'.

Signed-off-by: Henrique Vicente <henriquevicente@gmail.com>