Enable template_row to use external screens on_tap

The on_tap and on_delete methods on a template_row will set this blocks to the template children too.
It is then possible to add/edit/remove a more complex item in a new window.

```ruby
    data = {
      sections: [{
        title: "Shop",
        rows: [{
          title: "Name",
          key: :name,
          type: :string,
          value: "MyShop"
          }]
      },{
        title: "Buyed Items",
        rows: [{
          title: "Add Item",
          key: :items,
          type: :template,
          value:  [Item.new("Item1"), Item.new("Item2")],
          template: {
                title:       "",
                type:        :object,
                editable:    false,
                placeholder: 'Enter here',
                indented:    true,
                deletable:   true
              }
          }]
      }]
    }
    data = Formotion::Form.new(data)
    row = data.row(:items)
    row.on_tap do |row|
      obj = row.value.is_a?(Item) ? row.value : nil
      # use ProMotion to open/edit the Item object
      open ItemScreen.new(item: obj, item_editable: obj ? false : true, nav_bar: true), modal: true
    end
```

Example see button "Shop" in https://github.com/rheoli/formotion-example