fix nested grids with different acceptWidgets class

[adumesny]

Description

• fix for Adding widget to nested grid: Cannot set property 'isOutOfGrid' of undefined #1143
• added sample test where parent and nested grid have different drop class support
• while this prevents wrong class item from being dropped on certain grid

this.dd
  .droppable(self.container, {
    accept: function(el) {
      el = $(el);
      var node = el.data('_gridstack_node');
      if (node && node._grid === self) {
        return false;
      }
      return el.is(self.opts.acceptWidgets === true ? '.grid-stack-item' : self.opts.acceptWidgets);
    }
  })

the problem was that dropover|dropout|drop handlers did NOT return false (was handled) so the parent grid was getting the bubbled event, causing error

Checklist

• Created tests which fail without the change (if possible)
• All tests passing (yarn test)
• Extended the README / documentation, if necessary

[coveralls]

Coverage decreased (-0.1%) to 68.146% when pulling 5aae305 on adumesny:develop into b30655d on gridstack:develop.