Dropdown actions for index

When adding many custom actions using index as table you get a large column for actions.  This adds a 'dropdown' option to actions method

Some Examples:

To list default actions and customs actions:
```ruby
index do
  selectable_column
  column :title
  actions dropdown: true do |post|
    item "Preview", admin_preview_post_path(post)
  end
end
```

To list custom actions without default_actions:

```ruby
index do
  selectable_column
  column :title
  actions dropdown: true, default: false do |post|
    item "Preview", admin_preview_post_path(post)
  end
end
```