Change the target for reodering of layers. This makes sure that groups
stay together in the layer stack. Layers of a group cannot move outside of
the group.
@param {Object} state Current state.
@param {Object} action Action to handle.

@returns {string|boolean} The new targetId or false if moving should be blocked.
function changeTarget(state, action) {
  const layerIdx = getLayerIndexById(state.layers, action.layerId);
  const layer = state.layers[layerIdx];
  const layerGroup = getGroup(layer);
  const targetIdx = getLayerIndexById(state.layers, action.targetId);
  if (layerGroup) {
    return false;
  }
  let i, ii;
  if (layerIdx < targetIdx) {
    // move up
    for (i = targetIdx + 1, ii = state.layers.length; i < ii; i++) {
      if (getGroup(state.layers[i]) === layerGroup) {
        return state.layers[i - 1].id;
      }
    }
    return state.layers[ii - 1].id;
  } else {
    // move down
    for (i = targetIdx - 1; i >= 0; i--) {
      if (getGroup(state.layers[i]) === layerGroup) {
        return state.layers[i + 1].id;
      }
    }
    return state.layers[0].id;
  }
}