Add mirror property to Image, BorderImage and AnimatedImage elements

In right-to-left locales most images are not mirrored, but some directional icons should be. Using negative scale transform for mirroring icons already works for Image, AnimatedImage and BorderImage elements, but for convenience it would be nice to be able to provide a simple boolean property developers could use for mirroring. See parent task QTBUG-11042 for more info.

Image {
    id: image; source: "arrow.png"
    mirror: true
}