Support 'symbolic' themed icons

Support 'symbolic' themed icons

Add gtk_icon_info_load_symbolic() to load symbolic icons, and
theme their background/foreground colours to match the
colours used in the theme.

Adds the gtk_icon_info_load_symbolic() function, explicit
support in GtkImage and GtkCellRendererPixbuf, and test cases
for those 2 widgets.

With help from Bastien Nocera <hadess@hadess.net>

https://bugzilla.gnome.org/show_bug.cgi?id=614711