AUI-1249 Fix Tree alignment on Bootstrap 3
AUI-1251 Add min height to serve as a clearance for making space for the form builder toolbar action buttons

Signed-off-by: Henrique Vicente <henriquevicente@gmail.com>
AUI-1251 Update class name to use the css prefix 'btn' (now used by Bootstrap)

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