Add support for hiding visibility of an action from the Toolbar

[gobrightspot]

One thing we currently cannot do with this package, is use the default Nova Action visibility properties, (e.g. public $showOnIndex = false;) to effect the DetachedAction button in order to hide it on the Index view.

We may need to add a property and helper method to DetachedAction in order to set the visibility for the Vue component:

<?php

namespace Brightspot\Nova\Tools\DetachedActions;

use Laravel\Nova\Actions\Action;
use Laravel\Nova\Actions\ActionMethod;
use Laravel\Nova\Exceptions\MissingActionHandlerException;
use Laravel\Nova\Http\Requests\ActionRequest;
use Laravel\Nova\Nova;

abstract class DetachedAction extends Action
{
...
    /**
     * Indicates if this action is only available on the custom index toolbar.
     *
     * @var bool
     */
    public $showOnIndexToolbar = true;

    /**
     * Determine if the action is to be shown on the custom index toolbar.
     *
     * @return bool
     */
    public function shownOnIndexToolbar()
    {
        return $this->showOnIndexToolbar;
    }

    /**
     * Prepare the action for JSON serialization.
     *
     * @return array
     */
    public function jsonSerialize()
    {
        return array_merge([
            'detachedAction' => true,
            'label' => $this->label(),
            'showOnIndexToolbar' => $this->shownOnIndexToolbar(),
        ], parent::jsonSerialize(), $this->meta());
    }
...