Add Qt::AA_UseHighDpiPixmaps.

Setting this attribute enables QIcon::pixmap() to return
high-dpi pixmaps when running on "retina" type displays.

This requires an opt-in flag since the returned pixmap
can be larger than the requested size, which is a change
in previous documented behaviour that can break existing
code.
Make QIcon generate high-dpi pixmaps.

QIcon::pixmap() can now return pixmaps that are larger
than the requested size. This is a behavior change
that may require updating existing application code.

Update the existing pixmap() and actualSize() functions
to return high-dpi pixmaps when Qt::AA_UseHighDpiPixmaps
is set.
Update higdpi manual test.

(sync from Qt 5)