Abstract action events from ListScreen

[dincozdemir]

These functions in ListScreen shouldn't be aware of the event model (type, payload). We should provide corresponding functions in the action creators.

onAdd = item => {
    this.props.pushShoppinglistEventToFirestore({
      type: SHOPPING_LIST.INCREMENT_QUANTITY,
      payload: item
    });
  };

onSubtract ...
onToggle ...
onNewItem ...
onRemoveItem ...