Enhance srcset method

[nilshoerrmann]

Describe the PR

When discussing #1520 with @distantnative, I noticed that Kirby's srcset method is limited to resizing and doesn't take additional options for cropping or other image manipulations. This pull request intends to add this by switching from the resize to the thumb method for url generation.

The $sizes variable now accepts a new array structure moving the image condition to the key and the image options to the value:

$image->srcset([
    '1x' => [
        'width' => 38,
        'height' => 38,
        'crop' => 'center'
    ],
    '2x' => [
        'width' => 76,
        'height' => 76,
        'crop' => 'center'
    ],
    '3x' => [
        'width' => 152,
        'height' => 152,
        'crop' => 'center'
    ]
]);

The old condition syntax

$image->srcset([
  800  => '1x',
  1600 => '1.5x'
]);

still works for compatibility reasons but should be replaced in the docs with the more powerful proposed syntax in this pull request in my eyes.

The simple syntax of defining a plain array of widths has not been changed.

Related issues

• [Panel] Blurry images in image section #1520
• Support Cropping in $image->srcset() ideas#242

Todos

• Add unit tests for fixed bug/feature
• Pass all unit tests
• Fix code style issues with CS fixer and composer fix
• If needeed, in-code documentation (DocBlocks etc.)

👆 I have no idea what is really needed from this list and how it's done.

[bastianallgeier]

I just merged this manually! Thanks for the great PR