Wrong type used for desktop notification hints

Created by: yol

The Desktop Notifications Specification at https://developer.gnome.org/notification-spec/ requires that hints use specific types in the hashmap. Specifically, urgency has to be of unsigned byte type. hamster simply calls the notify function with an untyped python hashmap which causes all numerical entries to be created as standard 4-byte signed integer. This causes problems with notification daemon implementations that require the types to be set correctly (notably mate-notification-daemon).