support PDF and other multi-frame image formats in QIcon

Since we treat PDF as an image format, alongside SVG, it should be possible to load icons from this format. The trick is to come up with a way to load icons by name rather than by page number. But PDF files can use arbitrary strings as page numbers anyway; so the convention could be to use the freedesktop.org icon name, e.g. document-open, as the page number; or perhaps better, use a tree of bookmarks/contents so that they can be organized hierarchically.