[4.1] Clean up Request usage inside CrudController and CrudPanel

[tabacitu]

In Backpack 4.0, when working inside a ProductCrudController we had:

• $this->request
• $this->crud->request

which is superfluous and confusing, since:

• $this->request never gets overwritten, and is unavailable inside the CrudPanel object; so it's the same exact thing like using Laravel's request() helper - which I'd argue is cleaner and more intuitive (it looks like a global object); so let's use request() instead of $this->request which does nothing extra;
• $this->crud->request has a setter ($this->crud->setRequest()), but no getter - it's used as a property, which makes it difficult to change what it does;

Needs to be documented

• inside your ProductCrudControllers, if you're using $this->request, please replace it with request(); that CrudController property is no longer available, since it basically did the same thing as request();
• inside your ProductCrudControllers, if you're using $this->crud->request, please replace it with its setter or getter ($this->crud->getRequest() or $this->crud->setRequest()); the property is now protected;

[scrutinizer-notifier]

A new inspection was created.

[tabacitu]

Merged into 4.1 - PR #2508
Documented.