Make Image element support detection of suitable extension

In QImage / QImageReader, if you pass the path of a file that does not exist, it will not immediately error out, but try to append the different known, supported file extensions to the path to see if any of them exist.

In Quick, such functionality does not currently exist. But it would be especially useful for compressed textures, as it would allow the user to have a single URL which would automatically resolve to a compressed texture container when running on target hardware, but which would fall back to e.g. PNG when just testing on the desktop.

The idea would be that the image loader in Qt Quick would first try the supported containers for compressed textures, and if none exist, then it would use QImage to find the image. Thus it would both fill the a feature gap between QImage and Image, and it is a elegant solution to the problem of having one code base that would run seamlessly with different target hardware.