Convert afterFetch() method to an event handler for model.afterFetch

[mjauvin]

This change allows a plugin to handle the model.afterFetch event AFTER the Model handler.

The current code uses the afterFetch() method which always runs AFTER any event handler.

ref. https://github.com/wintercms/storm/blob/develop/src/Database/Model.php#L162-L168

The alternative would be to modify the code referenced above in the base Database/Model, but that could lead to breakage in backward compatibility.

[damsfx]

My test did not work.

Steps to reproduce :

• New installation of WinterCMS
• Edit the Demo theme to include a field

form:
   fields:
       header_images:
           label: Header images
           span: left
           type: fileupload
           mode: image
           imageWidth: 200
           imageHeight: 200

When going to theme settings, an error occurs :

Model 'Cms\Models\ThemeData' does not contain a definition for 'header_images'.

[mjauvin]

Closed in favor of #495