Manage built-in types and differentiate them from user-defined types

Current concept:

• user defines his mail types
• a "mail" type is automatically created, and assigned to loaded emails only if no other type matches

Target concept:

• user defines his mail types
• some built-in types are automatically created (having "builtin"="yes") : "mail", "calendar", and could be extended over time
• loaded mails are automatically assigned a "builtin type" (mail/calendar), and a "type" (depending on user definitions) at loading time. These are 2 different fields instead of a unique one.
• types and builtin types are managed from the same ui, but only "icon" field can be updated for builtin types. There are no matchers for builtin types.