GtkIconInfo: Implement paintable

GtkIconInfo: Implement paintable

This makes GtkIconInfo directly implement paintable by loading
the icon as needed. This is done in a blocking fashion for now, but
could be made more async in the future.

It also means we can't return errors to the called, but I doubt
anyone actually does anything useful with them other than showing
nothing (which we already do).

This also changes a fringe behaviour for unthemed icons. They used to
be never scaled down, but that means we can't tell without i/o the
size of the paintable. Since this is the only case we can't know the
size i took an executive decision and removed that behaviour. I don't
think picking some arbitrary much larger than requested size is ever
right, nor do i think using GtkIconTheme with unthemed icons is overly
useful. If you want to display some random non-iconish image, use
GtkImage instead.