autoload library directories

The vendor directory is managed by composer.  It's ignored by git and should not be 'polluted' with non-managed directories.  So, I've added `library` folders for both Admin and Catalog.  They can be used for core ZC code as well as required 3rd-party code, such as Aura - which I've included.  Additionally, local code can also live there, i.e. `Texdc\Admin\MyFancyWidget` in `admin/library/texdc/my-fancy-widget` - if not managed by composer.

The autoload config has been moved to `admin/includes/config/config.autoload.php` so that it can be managed independently and overridden easily.  There are 2 new directory constants, `DIR_ADMIN_LIBRARY` and `DIR_CATALOG_LIBRARY`.  I can update the documentation on how to use and override these config files, when necessary.

Lastly, I believe that the `zcDashboardWidgetBase` class was inadvertently left out of #95.  It's renamed to `ZenCart\Admin\DashboardWidget\AbstractWidget`.

Thanks again for adding namespaces and autoloading :100: .  This is a great start to refining and modernizing ZC code!