Update crud-controller.stub for Backpack v4

[tabacitu]

Description

Changes the crud-controller.stub to generate Backpack v4 controllers.

Motivation and context

The package did not provide a way to generate Backpack v4 controllers.

How has this been tested?

No, it has not! PLEASE DON'T MERGE THIS YET. Before we merge this we need:

• Allow installation on Laravel 6.1 and 6.2 #39 to be merged;
• your reply on the matter below;

Another IMPORTANT problem with this PR is that there's no backpack/crud requirement to webfactor/laravel-generators. This means that it assumes the user is running a certain Backpack version that is compatible with the stub. The current PR assumes they're running Backpack v4, since that's the current version, that everybody is installing (Solution 1).

(Solution 2) A different way to approach this would be to have two controller stubs: crud3-controller.stub and crud4-controller.stub, and before generating the crud controller, to see which Backpack version is installed and use the correct Stub. This is a little more complicated, but not too complicated. It would probably require adding a dependency (ocramius/package-versions) to check the currently installed version.

I don't know if it's really necessary though - why support Backpack 3.x when 4.x is here. What's you position on this? Solution 1 or Solution 2?

Screenshots (if appropriate)

Types of changes

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

Go over all the following points, and put an x in all the boxes that apply.

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