Add support for Variant Presets

[kdambekalns]

I'd have a go at adding support for "variant presets" to Kaleidoscope. But…

First question would be: What to do about the name clash with the existing (thumbnail) preset option on ImageSource?

Should there be a new option variant? And should that be given as <PresetIdentifier>::<VariantIdentifier> or rather as two new options variantPreset and variant?

We could also "reuse" the preset option and take it as variant preset when given with :: in the identifier… too magic?

Any opinions here, @mficzel, @robertlemke, @kitsunet?

[mficzel]

@kdambekalns i would go with the name VariantPreset as the existing presets are working differently and this is what it is called in the configuration. (maybe we should rename the key "preset" to "thumbnailPreset" alongside).

Primarily the ImageSourceHelperInterface has to be extended for that (not really sure wether to use a combined identifier or two strings here):

interface ImageSourceHelperInterface {
    public function applyVariantPreset(string $group, string $variant) : 
ImageSourceHelperInterface;
}

While the function has to be implemented in all ImageSourceHelpers it can only have an effect in the scalable ones (Asset and Dummy).

In the assetImageSource the asset variant will obviously be switched. In addition the target dimensions have to be adjusted for Asset- and DummyImageSources from the variant configuration which is the part i consider most complicated.

The idea is that it should be possible to decide on the actually used variant on the presentation side which will be especially helpful for picture-tags.

On top of this it obviously makes sense add a variantPreset option to the Sitegeist.Kaleidoscope:AssetImageSource and Sitegeist.Kaleidoscope:AssetImageSource that internally trigger a call of applyVariantPreset(). That will make accessing variants on the integration side as easy as on the presentation side.

[mficzel]

@kdambekalns this is released as kaleidoscope 5.1.0. I even managed to make this 4.3 and 5.0 compatible by checking if (FLOW_VERSION_BRANCH == '5.3') before sending cache headers