Add plugin for animated Windows cursors (ANI)

• Supports QImageIOHandler::Size for getting the image size without reading a full frame
• Supports the QImageIOHandler::Name (mapped to INAM) and QImageIOHandler::Description (mapped to IART (artist)) properties
• Supports per-frame animation delay (rate field)
• Supports arbitrary frame order (seq field), fakes duplicated frames for Qt so you get a linear set of images.

Rendering the actual embedded cursor pixmap is delegated to Qt's built-in "cur" plugin.

Thumbnail (after adjusting windows image thumbnailer to also load ANIs)

Viewing its first frame in Kolourpaint (there's no write support)

Using it from an AnimatedImage

qtimageioqni-2020-12-25_23.36.16