Render stock:// as pixbufs and pass as data URIs

Render stock:// as pixbufs and pass as data URIs

Stock icons do not match filenames in many icon themes and the
appropriate sizes may not be available. Thus we now always
render the icon through the theme engine and encode it as a
data URI with BASE64/ PNG.

As a side effect, we use stock sizes now instead of pixel sizes,
where 1 means menu size, 4 means button size and 6 dialog size;
the value 16 is translated to 4 to keep existing files working.