Change GtkIconSize to int in params/return values

Change GtkIconSize to int in params/return values

GtkIconSize is an extensible enumeration (via
gtk_icon_size_register()), so methods that claim to take/return a
GtkIconSize need to actually use "int" to work correctly with bindings
that are strict about enum values.

https://bugzilla.gnome.org/show_bug.cgi?id=604895