Implement better highdpi handling in QImageReader

Right now @2x handling is spread in several places (qicon, qstylesheet, user code, etc.)

QImageReader seems a reasonable candidate to partly centralize this behavior:

• add a autoDevicePixelRatio()/setAutoDevicePixelRatio(bool) method pair.
• add AutoDevicePixelRatio to QImageIOHandler::ImageOption
• if the IO handler supports vector scaling (like the SVG handler), autoDevicePixelRatio will be handled by the IO handler, scaling the svg image to the right dpr
• otherwise, QImageReader will look for an @2x file, and if it finds it, it will read that and set the device pixel ratio to 2.0
• QPixmap::QPixmap(const QString& fileName, const char *format, Qt::ImageConversionFlags flags), through QPlatformPixmap::fromFile(), will set autoDevicePixelRatio to true. This is because QPixmap is meant to be used for displaying images on screen.
• QImage(const QString & fileName, const char * format = 0) will NOT set autoDevicePixelRatio to true
• if autoDevicePixelRatio is false, QImageReader will not set the device pixel ratio on the image, irrespectively of the @2x (see QTBUG-38485)

Does this make sense? I could work on this if there is some consensus