Reuse rendered symbolic icons

Reuse rendered symbolic icons

With the previous commit all loads of the same icon will share a single
GtkIconInfo, which typicallty means the pixbuf is shared via Info->pixbuf.

However, atm we don't share symbolic icons, which causes these to be re-read
and re-parsed every time. This is especially bad if the icon is used many times
in some form of list. So, we cache the pixbufs and reuse them.

https://bugzilla.gnome.org/show_bug.cgi?id=689081