Mention GtkIconSize in all docs where the argument/return is annotated as taking...

Mention GtkIconSize in all docs where the argument/return is annotated as taking an int instead of GtkIconSize

In https://bugzilla.gnome.org/show_bug.cgi?id=601425 the annotations
were changed to int as they not only take the predefined enum values
but also user defined values registered through gtk_icon_size_register()

As a result the typelib doesn't contain any information about
GtkIconSize for those arguments and the Python docstring only
shows the corresponding Python type "int".

This changes the argument docs to mention the type explicitly
so the Python doc generator can add a link to Gtk.IconSize
which contains the most useful predefined values.

https://bugzilla.gnome.org/show_bug.cgi?id=757411