Picture helper improvements

[ErikHen]

• Picture helper: Possible to create Webp versions for other formats than jpg. Add formats to the CreateWebpForFormat array in the Image Type.
• Picture helper: Webp versions of png images will lossless by default. Can be changed by setting CreateLosslessWebpForPng in the Image Type.
• Picture helper: decoding="async" is now by default added to the img element. Can be changed by setting ImageDecodingin the Image Type.
• Picture helper: DefaultImgWidth is now optional. Will use the largest SrcSetWidth if not set.