Stock: Add 'Gambas' and 'Gambas-Mono' to the icon theme list.

[DEVELOPMENT ENVIRONMENT]
* NEW: Option dialog: Remove 'Gambas' and 'Gambas-Mono' from the theme list as they are now returned by the Stock.Themes property.

[GB.FORM]
* NEW: Stock: Add 'Gambas' and 'Gambas-Mono' to the icon theme list.