IconCache: Keep a ref on the GtkIconData

IconCache: Keep a ref on the GtkIconData

The icon data in GttkIconInfo->data is currently owned by the
IconThemeDir->icon_data hashtable. However, on e.g. a theme change
blow_themes() destroys the dirs and thus the data, meaning any
outstanding GtkIconInfo points to stale data.

We solve this by adding a refcount to GtkIconData and reffing it
from GtkIconInfo.

https://bugzilla.gnome.org/show_bug.cgi?id=702598