[6.x] Pinned Actions

[duncanmcclean]

This pull request introduces the concept "pinned actions" - essentially actions that are visible without needing to open the dropdown.

On one of my sites, the most common action content editors make is syncing videos from YouTube.

Right now, they have to open the actions dropdown and trigger the action from there. It would be nice if it was slightly more obvious - like "Create Entry" is.

This PR makes that possible - simply set the $pinned property in your action and it'll render as a button where it makes sense.

class SyncFromYouTube extends Action
{
    protected static $title = 'Sync from YouTube';

+    protected $pinned = true;

    public function icon(): string
    {
        return 'sync';
    }
    
    public function run($items, $values)
    {
        // ...
    }
}