Improve preloader animation

[dfsmania]

Question	Answer
Issue or Enhancement	Enhancement
License	MIT

What's in this PR?

Add some improvements to the preloader animation:

• Add style="animation-iteration-count:infinite;" so the animation on the default content logo now plays indefinitely.

• Use the img-circle class on the default logo content to get a circled styled logo.

• A new configuration option (mode) is now available so the preloader may be attached into to the content-wrapper element instead of being fullscreen.

• The default preloader content with a logo image may now be replaced completely using a new yielded section named preloader. At next, an example to replace the default preloader logo with a Font Awesome spinner icon:

  @extends('adminlte::page')
  
  ...
  
  {{-- Preloader replacement --}}
  
  @section('preloader')
      <i class="fas fa-4x fa-spin fa-spinner text-secondary"></i>
  @stop

Checklist

• I tested these changes.
• Create a PreloaderHelper class to split-out LayoutHelper.
• Update related docs on Wiki.