sequence_item.pt fixes

- 'row' (not 'container') is the class for the column container in bs3

- add 'form-group' class to provide vertical margin between sequence
  items

- 'col-xs-*' instead of 'col-md-*' so that close button and drag handle
   always get their own column
Merge pull request #186 from dairiki/sequence-item

deform2: sequence_item.pt fixups